I found it almost impossible to break up if returning to an ex for sex. I left my ex-husband probably 4 times before it was finally over but had a boyfriend by then, but trouble with the boyfriend led me back to my ex-husband, then trouble with the ex-husband led me back to my boyfriend, which was also a bad relationship. I finally got the ex-boyfriend out of my system because I CAN'T FIND HIM (he moved, etc) and my ex-husband got a girlfriend. Well the girlfriend didn't work and the ex-husband came back to me. But I still pine for the ex-boyfriend so got a new lover (to use) to forget him. I do NOT suggest continuing a sexual relationship with any ex.
Glad you lived to tell the tale and warn others!