I'm sorry that you are going through this. Maybe she's just scared of how she will get by on her own. Maybe she's been talking too much to other women.
My ex- and I had a very amicable divorce, both our boys were over 18, so no child support to worry about. We split everything 50/50; divided the house equitably. He does pay me alimony because I mainly worked part-time in order to be home more and take care of the children, but we came up with a figure that we could both live with. Hopefully you can do the same.
We still spend time together, do things together, help each other out, etc. I was so suprised the negative comments I get from people- that they can't believe I get along with my ex-. Aren't we "supposed" to be fighting, mad at each other, etc.
Maybe she's buying into the assumptions and comments from others that "she's supposed to stick it to you", etc.
I would suggest that you consult with an attorney and find out what the laws are where you live and what you are entitled to. Also, can the two of you talk it out, go see a counselor together?
I will tell you that our boys are very grateful that we all get along. The four of us spend holidays/birthdays/family dinners together. They have actually told us that they are glad we get along. It will be so much better for your children if both of you can do this amicably and I hope she sees that.