Sounds to me like "Flight to Forever" by Poul Anderson (first published in Super Science Stories, November 1950, available at the Internet Archive). It doesn't match your description perfectly, but at least it is a time-travel story.
I believe it was a full novel,
Not quite. The ISFDB calls it a novella, which I think is the same as a short novel. I read it in an anthology called Year's Best Science Fiction Novels 1952.
that I must have read as a preteen, so it can't be newer than the late '90s or early 2000s.
It's old enough all right, first published in Super Science Stories, November 1950. In the 1990s you might have read it in Poul Anderson's collection Alight in the Void
I don't remember much, but I think it was about a man who travels forward in time and gets stuck there,
The hero of Anderson's story goes 100 years ahead in his time machine, and then he finds he can't get back to his own time, because his machine won't go in reverse. More precisely, it can go backwards in time, but not more than about 70 years. So he decides to keep going farther into the future, until he finds a society advanced enough to be able to send him back. E.g. in 4100 A.D.:
"Can you help us?" asked Saunders bluntly. "Can you fix our machine so it will reverse?
"Alas, no. I am afraid our physics holds no hope for you. I can consult the experts, but I am sure there has been no change in spatiotemporal theory since Priogan's reformulation. According to it, the energy needed to travel into the past increases tremendously with the period covered. The deformation of world lines, you see. Beyond a period of about seventy years, infinite energy is required."
eventually falling in love with a tribal woman. . . . I think the woman may also have been a princess or daughter of a chief or something.
In 50,000 A.D. he falls in love with Taury the Red, Empress of the Galaxy. The Empire has seen better days:
Hunda shrugged. "It's a harmless game, isn't it? At that, you know, we are the Empire—legally. Taury is a direct descendant of Maurco the Doomer, last Emperor to be anointed according to the proper forms. Of course, that was five thousand years ago, and Maurco had only three systems left then, but the law is clear. These hundred or more barbarian pretenders, human and otherwise, haven't the shadow of a real claim to the title."
And Taury is a warrior queen; she leads her space fleet into battle against the barbarians:
"In a few minutes," said Taury quietly. "In just a few minutes, Martin."
She paced back from the viewport, lithe and restless as a tigress. The cold white starlight glittered in her eyes. A red cloak swirled about the strong, deep curves of her body, a Sunburst helmet sat proudly on her bronze-bright hair. Saunders thought how beautiful she was—by all the gods, how beautiful!
She smiled at him. "It is your doing, Martin," she said. "You came from the past just to bring us hope. It's enough to make one believe in destiny." She took his hand. "But of course it's not the hope you wanted. This won't get you back home."
"It doesn't matter," he said.
"It does, Martin. But—may I say it? I'm still glad of it. Not only for the sake of the Empire, but—"
A voice rattled over the bridge communicator. "Ultrawave to bridge. The enemy is sending us a message, your majesty. Shall I send it up to you?"
"Of course." Taury switched on the bridge screen.
He lives there for some time
About a year:
The last months rose in his mind, a bewilderment of memory. After the surrender of the Anvardian fleet, the Imperials had gone under their escort directly to Canopus and thence through the Anvardian empire. And chief after chief, now that Ruulthan was dead and Taury had shown she could win a greater mastery than his, pledged allegiance to her.
Hunda was still out there with Belgotai, fighting a stubborn Anvardian earl. The Dreamer was in the great Polarian System, toiling at readjustment. It would be necessary, of course, for the Imperial capital to move from isolated Sol to central Polaris, and Taury did not think she would ever have time or opportunity to visit Earth again.
until they are going to be married, but then his time machine is fixed and he may go back home.
Not exactly. The marriage seemed to be in the offing, but then Saunders is waylaid by a rival, bound and gagged and exiled to the future in his own time machine:
Saunders tried to move, snarling incoherently through his gag. Vargor shook his head. "Oh, no, Martin. I can't risk letting you make an outcry. If I'm to do evil, I'll at least do a competent job of it.
"I love Taury, you see. I've loved her ever since I first met her, when I came from the stars with a fighting fleet to her father's court and saw her standing there with the frost crackling through her hair and those gray eyes shining at me. I love her so it's like a pain in me. I can't be away from her, I'd pull down the cosmos for her sake. And I thought she was slowly coming to love me.
"And tonight I saw you two on the balcony, and I knew I'd lost. Only I can't give up! Our breed has fought the Galaxy for a dream, Martin—it's not in us ever to stop fighting while life is in us. Fighting by any means, for whatever is dear and precious—but fighting!"
He finally gets his time machine fixed by the godlike beings who rule Earth in 4,000,000 A.D. They give it an upgrade so it can go "around the Horn" of circular time and back to when he started:
He was in a city. But it was not such a city as he had ever seen or imagined, he couldn't follow the wild geometry of the titanic structures that loomed about him and they were never the same. The place throbbed and pulsed with incredible forces, it wavered and blurred in a strangely unreal light. Great devastating energies flashed and roared around him—lightning come to Earth. The air hissed and stung with their booming passage.
The thought was a shout filling his skull, blazing along his nerves, too mighty a thought for his stunned brain to do more than grope after meaning:
CREATURE FROM OUT OF TIME, LEAVE THIS PLACE AT ONCE OR THE FORCES WE USE WILL DESTROY YOU!
Through and through him that mental vision seared, down to the very molecules of his brain, his life lay open to Them in a white flame of incandescence.
Can you help me? he cried to the gods. Can you send me back through time?
MAN, THERE IS NO WAY TO TRAVEL FAR BACKWARD IN TIME, IT IS INHERENTLY IMPOSSIBLE. YOU MUST GO ON TO THE VERY END OF THE UNIVERSE, AND BEYOND THE END, BECAUSE THAT WAY LIES—
He screamed with the pain of unendurably great thought and concept filling his human brain.
GO ON, MAN, GO ON! BUT YOU CANNOT SURVIVE IN THAT MACHINE AS IT IS. I WILL CHANGE IT FOR YOU . . . GO!
The time projector started again by itself. Saunders fell forward into a darkness that roared and flashed.
