you are doing a lot of assuming here. Don't. Do not believe anything you hear and only half of what you see. Do not believe she is happy, she only thinks she is happy. And she is acting happy. And would you not like the fact that your kids would be living with a happy person rather than a miserable depressed one?
I know what you are saying, I know how you are feeling. I've been there, we've all been there.
Stop with overthinking. Now is the time to be still, to heal and to concentrate on yourself and the kids.