Sex with an ex

Soooooo... I did something bad.. I think.. I slept with my ex. (He is my 1 and only ex as i was with him for 10ish years. First love).Not gonna lie, we've slept together 3 times in the nearly 2 year split. I'm actually annoyed with myself for doing it. He literally broke my heart and can be an absolute arsehole. We have to stay in contact for our child. So I'm not sure why I would even want to... but I do. Plus a girls got needs haha. Not even sure why I'm writing this here tbh. We slept together recently and he kept making little comments which actually made me feel pretty shitty. I don't think he realised (or maybe he did)... and maybe it just me because I am insecure and self conscious all of a sudden. I did actually mention this to him but he just said he was kidding and to not be so serious.. I don't know why I've took it to heart tho. I'm angry at myself as I don't know why it should bother me so much. Or why it hurt. I knew he didn't care for me as I did him while we were together as his actions spoke volumes. I know I'm not in love with him anymore. I know I still care as I'm a very compassionate person and care about everyone. But why oh why has it affected me so? Should I stop? (Even writing this now, I could easily have sex again). What is wrong with me .