very very hard.
But, write her a text, an email, a letter, put on Facebook for all to see, all if them, and get your ex DH to give one , that say this:
Whatever you may think, I love you and have always tried to do my best for you. I shall always love you, no matter what. Go safely, love Mum.
No more, no less ( change the go safely but leave it simple) .
Then leave it. With time, maturity, experience she may come round. But St the moment she won't.
You send presents for Xmas and birthday to your ex DH or foster family. And you reiterate that simple message. No more, no less.
And you start your life again. With a broken heart but a calm exterior.