Why is it when I came out as ftm trans to my ex 6 years ago 8 months after she split up with me, she was absolutely fine about it and told me she’s proud of me and will be there for me. And now she said she hates me for not telling her when we were together?

Sounds like a case of:
"WILLY NO COME BACK AGAIN"
*********
But seriously , no-one on ASK knows you ; how can they possibly analyse ?
So, she assumed she was in a female-gay relationship with you.
As time passes , it will be dawning upon her . . . . she was in a relationship with a male (will feel like)
That must be really freaking her out . . . .
"WILLY NO COME BACK AGAIN"
*********
But seriously , no-one on ASK knows you ; how can they possibly analyse ?
So, she assumed she was in a female-gay relationship with you.
As time passes , it will be dawning upon her . . . . she was in a relationship with a male (will feel like)
That must be really freaking her out . . . .