Validating is about making the other person feel heard, not about admitting they were right.
"That must feel terrible to think that" "I'm sorry that you feel that way" "I didn't mean that in that way" " Do you feel (X,Y,Z)?" "It was not my intention for you to feel that way"
Others can recommend more sentences.
When you are defending yourself, you are invalidating what she's saying. Not that you can't defend yourself, but not always at the same time you are validating how she feels.
All this being said - it's possible that she is not an emotionally mature enough person to make this work. That's something you will find out in the future. But I do think that trying to make it work right now, until your baby is older, is probably a good strategy (check with your lawyer). Continuing to go to therapy together may be your best chance if this relationship is salvageable.
