Dealing with my fiancè's baby mama drama

My fiancè:s baby mama/ex wife has made life so difficult. They have 2 kids together. One is 15 and the other is 7. His 15 year old lives with us full time but they share custody of the 7 year old. She's not over him at all and has harassed both of us for months. To the point they now have to do their custody exchanges at the courthouse. She has tried so hard to make me feel insecure about our relationship. She made a whole Facebook post bashing me. Saying she saw the engagement ring and can tell it's cheap. And hers was 16,000. Mine was 1,000 so that did have me a little insecure. She tried to make me insecure by saying in this Facebook post how much closer he was to her than he will ever be with me and ask if I knew about certain things and some of those things he didn't tell me. She asked in this post did he open up about his rape. And no he didn't. That's how I found out. But honestly who posts that on Facebook? Which really upset him and I didn't even bother asking why he never told me. She keeps telling me that no woman will ever be able to beat the relationship of a man and the mother of his children. I know I should not feel insecure or inferior to her but she's pushed me so much I can't help but feel it start to effect me.