The book is definitely Scatterlings by Isobelle Carmody. I've tried to answer all of your recollections with quotes from the book - you've got a fantastic memory! Hope this helps.
The book starts with a girl waking up in a vehicle crash.
The girl’s name is Merlin. She wakes up in the wreck of what she thinks is an ambulance but is actually a “flier”
She was in the back of a van surrounded by blankets, pieces of broken glass and unfamiliar implements. Through a dark tinted window under her, running the length of the cabin she could see foliage pressed against the glass. A matching window above showed the sky. Clearly she had been in an accident and the van had overturned.
Ambulance, she thought dazedly. I was in the back of an ambulance so there must have been something wrong with me before the accident.
Then later on…
“Pity you didn’t come sooner,” Ford said absently. “One of Sear’s old traps netted a Citizen gods’ flier.”
She explores the planet - she walks through a city that has been overtaken by plants/trees.
The city itself was a ruin…trees, once confined to circular grills had grown to a monstrous size, their serpentine roots twisting and writhing in all directions, cracking open what remained of sidewalks.
However she has flashbacks of the city with thousands of inhabitants, which obviously happened hundreds years ago.
Around her, the city seemed to reshape and reform before her eyes, assuming a ghostly familiarity…but her memory was of a vigorous, busy, populated city, not an ancient ruin.
She meets some tribe people and they have telepathic powers.
Again his lips did not move and Merlin realised, amazed, that he was reproaching her telepathically.
The tribe people are terrified of another humanoid species that lives on the planet - they are technically advanced but don't have telepathic powers.
The tribespeople are not really terrified – they just think of them as gods who have great powers. Thus the name “Citizen gods”.
“The Lord warden of our clan said we had no right to demand mindbond of a god; that a god is truth,” said another.
She accompanies the tribe to a large meeting in a valley and discovers that she also has telepathic powers, though she struggles to control them at first.
She discovers that she has a telepathic block like the ‘Citizen Gods’ at least at that point in the book. It’s only much closer to the end that she realises she has telepathic powers.
“Why do you refuse to Accept me? How do you close your mind like the Citizen gods?” she heard him ask.
The tribes people have some weird pseudo-religion where they send off tribesmen to the other species as a sacrifice and they are returned physically untouched but with there brains completely and irreversibly wiped of personality/memory.
Yep, but it’s more a combination of law and religion. And the sacrifices return pretty much like drooling idiots.
“The clans tithe children to the Citizen gods, just as they tithe to Conclave. This is for the good of the many. You know that. Before this, our people were hunted down and herded like cattle. Many died. It is for the best.”
And then later…
“You gave the boy up because it was the law, and because it was for the boy’s good as well…once chosen, he had the chance to become a Blessed Walker.”
It turns out that the Citizens actually created the religion to get the clanpeople to want to sacrifice their children.
“It was Sacha who came up with the idea of making the clanpeople want to come into the city. She designed a mythology which focused around the city and the Citizens, and she brought in and brainwashed a number of tribal wardens using a highly addictive drug to ensure they were obedient.”
She's really conflicted as she's not sure if she belongs with the tribes people (who she looks like and shares powers with) or with another species who she remembers in flashbacks.
Also, because not a lot of what the tribesmen do and how they think makes sense to her, and from what she hears, the Citizen gods sound more like her “kind”.
In the end she goes to seek out the other species.
Tthe tribespeople send her to the Citizen gods as part of the clan tithe. This is because a person called a “Rememberer” (a type of tribesperson who can see the future) told the clanspeople to do it.
The Remember raised her hand. “She must not die by the hand of the clans. That will bring death and ultimate disaster on us. She must go to the forbidden city. She must go to the Citizen gods. Her fate does not lie in clan hands.”
It turns out they found her as a baby and raised her...
“I don’t know who you were originally,” he said…Andrew found you wandering half starved and witless. I suppose your parents had been killed somehow. You were very young and the trauma of seeing your family die had taken away your senses. But that is only a guess.”
They need someone with telepathic powers to link in with their AI to launch a rescue vessel and get them off the planet.
Yes, because they had been woken from stasis too early but they couldn’t get to the emergency ship due to the paranoia of the people who put them to sleep in the first place (the scientists who developed the technology thought that another country might take over the ship). If the computer was touched, the entire city might have been destroyed.
“You see, the ship was set to be released from the force-field protecting it in twenty centuries, at the same time as we were to be wakened. But we woke five centuries too soon. The computer won’t let us near the ship for another five centuries.”
“[Andrew] believed that an outsider with telepathy might be able to reach the computer’s mind without having to touch it and alter the program so that the computer would switch off the machine.”
But the tribesmen they used couldn't cope with the technology and it wiped their brains.
“Those who come to us are tricked into attempting to access the computer telepathically. But for some reason, their minds are unable to cope. Andrew refuses to accept that his great plan might not work, and so the trying goes on.”
They thought they could raise her with technology and she wouldn't be so overwhelmed.
William looked up. “Yes. You were the next stage. You came to us without a mind, so there was nothing to burn away, but also nothing to think with. Andrew had the idea that if we could raise you as one of ourselves and give you a mind that would help you align with us, you would be able to access the computer where others couldn’t.”
But she didn't develop telepathic powers so they had staged the accident and implanted certain memories so she could meet the tribespeople and learn to use her powers from them.
The Citizens didn’t know if she had powers before she awoke – they just assumed that because she was a tribeperson she would. However the accident wasn’t staged and the Citizens had no intention of her finding the tribespeople.
”You were unhooked from the computer and taken by flier to a smaller dome where you were to be ‘found’ by Andrew and Sacha...you were never meant to see that there was life going outside the dome. You would live among us, and bond with us.”
It also turns out that the tribes people and the others are actually both descendants from the human race. Something had left earth inhospitable to life and the tribesmen had evolved from the people left exposed to the new conditions of earth, whereas the others had put themselves in stasis with a computer searching the stars for a new home. I think it ends with the girl successfully launching the ship.
Yes, she does launch the ship and you’re right about the stasis.
“Your clanpeople are the descendants many times removed of those people who were refused admission to the domes, and who somehow adapted to the poisons, the increased strength of the sun, and the heat. We are not descendants – we are members of the survivors who lived in the domes.”
Edit to add: Just remembered something about the scene where she used her new found telepathic power to connect to the computer. The reason the other people were damaged by the computer is because everyone in the tribe learned from birth to have their telepathic connections constantly open; they didn't really know how to shut it off.
When the protagonist connected with the computer she felt this huge wave of information flooding towards her and she instinctively shut down her telepathic link. She then realised that because the other telepaths wouldn't have had this ability, they had always been amazed at her ability to shut other people out of her thoughts. The "sacrifices" would have had all this information flood in at once and they couldn't cope with it... a bit like plugging something into a socket that had too higher voltage. She could slow down the information flow to a level that she could cope with.
At that same instant, she understood with dazzling clarity why the others had gailed. In fact, they had not failed to access the computer at all. It was what happened after access that destroyed their minds. The problem was that clanpeople were natural telepaths, never questioning their ability or looking at it as something that might be dangerous…but Merlin had woken with a mind bereft of memory. Her telepathy existed outside the framework and moral direction that ruled the clanfolk, for what she remembered was an older, more corrupt world. Her first reaction to telepathic contact had been to prevent Ford from accessing her mind. Instinctively, she had refused him entry, and from that moment she had gone on blocking out all other minds, regarding the telepathic ability she possessed with fear and distrust.
Her attitude to telepathy had created an ability to block and this saved her from the computer which, once accessed, simply dumped its vast reservoir of knowledge into the accessing mind, as it would into another computer.
Sidenote I'm less sure about: She may have had blonde hair and brown eyes... I think that was how they recognised her as a member of their own tribe... and the other species I think wore suits and underneath the suit were really pale.
These were decontamination suits since the Citizens inside the dome weren’t immune to the poisons outside.
“We caught one of them once. Stripped of those shiny skins they wear, he was white as the cooked flesh of a chooken…said he’d die without the shiny skin. He died the next day.”
Merlin looks like a tribesperson.
The hair around the face was shoulder-length, coarse and copper-coloured…dark gold skin…the eyes a peculiar yellow shade.