Ahhhh Newcomer - I'm so sorry. I haven't talked to my soon to be ex-in-laws. I wanted to let him have his own support system. If my MIL did this to me, I don't know how I'd feel. Probably like you do now. Nothing you can do but vent, and know you're doing what you need to, but still ....
Looking at you from the outside, I can clearly see that she doesn't want to hear from someone who knows exactly what her son is. It's not you. But, if I reverse it, the feeling would be two-fold. Understanding and confusion.
Try to step outside. Good for you for writing it out. It does help so much to not just write some things down, but send them out for all of us to see.
((((hugs))))