Why am I still attracted to my ex

I'm still attracted to my ex even after 4 years 2 kids a house and everything I'm very attracted to my ex I don't know what to do. I can't have sex with my kids dad and enjoy it I don't get into it I just wanted to be over and done with. He's good don't get me wrong I just am not attracted to him. I can't kiss or rub on him I don't like to kiss besides a quick little Peck When hes going to work He turns me off from touching me and I don't know why I have tried to get into it and I can't I've bought toys and everything I just can't but if my ex even looks at me i get turned on. Has anybody else had this happened to them I just want to feel normal I want to enjoy it with my kids dad instead of wanting my ex. I don't want this it sound wrong i dont see my ex besides running in to him randomly at the store or something maybe once a year. Its a new year and i want to do things right i just don't know how.