Why do I still feel like I have to be nice?

Had a week from hell with the ex.. Arguments, his name calling, me crying, him saying I destroy peoples lives, finding out he & my first ex are now in cahoots, (the guy he HATED b/c he never saw the kids, didnt pay child support etc..) Anyway, he takes the kids for an overnight yesterday & I think alright Regina, just try being civil & I find myself falling all over myself to be nice, when all he can do is comment on "how sexually frustrated I make him with my new rack" GOOD GRIEF.. then he drops the kids off & I give him a muffin to take to work for breakfast.. DId someone kidnap me in the middle of the night? Why am I being an ounce nice to this jerk? Yes I left, I was the leaver, but it took YEARS and tons of abuse to get out...I dont love him, I dont like him, but the "caretaker" in me still wants to fix things for everyone else so they can be okay with my decision to be happy.. WTF??? I dont know where Im going with this other than beating myself up & not understanding WHY I would feel the need to be nice to someone that calls me an F***ing C**t at least once a week...