After Louise has her solo experience with the Heptapod "Abbott," She tells Ian she knows why her husband left her. What did I miss?
Her husband left her because he found out that she knew about their daughters death but did nothing to try and prevent it.
Are you gonna leave me like Daddy did?
Louise snaps back to full attention on her daughter.
Hannah, honey, your father didn't leave you. You'll spend time with him this weekend.
He doesn't look at me the same way anymore.
I'm... That was my fault. I told him something he wasn't ready to hear.
Believe it or not, I know something that's going to happen. I can't explain how I know, I just do. When I shared it with daddy, he got real mad. Said I made the wrong choice.
It has to do with a real rare disease...
Louise then returns to the present and tells Ian her realisation.
I remembered something
Why my husband left me.
Louise wipes her eyes and struggles to find her game face.
Unfortunately, the source novella, Story of Your Life, doesn't address why Ian and Louise eventually split, suggesting it was merely an invention for the film.
The other questions touch on the dialogue but the answer, here, is really the crux of the movie itself. I added spoilers, since there's no way to talk about it otherwise
Louise is experiencing time in reverse. That's the gift of Abbott and his kind. Once you understand the language, you can "think outside of time". That's how Louise knows what to say to the Chinese general and stop the pointless fight that was coming.
As to the question
Louise's daughter has a rare and incurable disease. We know this because her daughter dies. But it's an incredible burden to bear to know what sorrow is to come, and yet to choose it anyways. But at some point she tells Ian she knows their daughter will die. But how would you feel to know that someone made a choice, involving you, that would one day lead to your heart being broken? For Ian, the answer is he felt anger. Anger that she went ahead in having a child she knew would die.
Her husband haven't left her, yet, but he will.
She had visions of her future self with her daughter being treated for cancer. In future, her husband, Ian leaves her when he comes to know about their daughter's illness. She knows this in present time already, and still chooses Ian as her life partner.
"If you could see your whole life laid out in front of you, would you change things?"
He left because he didn't agree with bringing their daughter into the world knowing she would die from cancer at so young an age. I'm guessing Hannah felt the difference because her dad couldn't look at her without feeling his heart break knowing they'd be losing her so soon. Amy Adams' character, though, chose to know her daughter and experience what time they had together despite knowing where that journey would lead. The time they had was worth the grief and pain and loss. It's a beautiful movie.
I think it’s a little bit deeper than that. Louise was so deeply in love with her daughter and cherished every moment that they had, even knowing the painful loss to come. And knowing how telling him is what destroyed their marriage, she has the opportunity to change that outcome, in the same why she was able to change the outcome with the Chinese General. The general is a parallel for her personal story.
In some ways she’s a bit like the Greek goddess Cassandra; knowing the future and knowing the futility of telling.
And I believe that her tenderness with Ian at the end is her coming to understand how difficult losing Hannah is going to be for him, and that she must be strong and keep the bad news to herself if she wants to be a part of his healing. It could be a form of torture but she chooses to cherish the journey. And to love. To give so much love.
Also brings up a profound question about choice. She made the choice for him, instead of with him, knowing full well he wouldn’t have had Hannah if he had known. How do you balance that? Making such a choice for the two people you love most in the world, and want to share this important journey with? Me? I would have blabbed and spoiled everything ;0/