Hooking up with an ex who is the father of your kids

OK so me and my kids dad were together for 4 years and two months ago I made the mistake of breaking up with him...I did because I didn't know if I was happy anymore I suffer from ptsd and depression so i go threw bouts of where I rethink and question everything but this was the first time I acted on it and actually said anything. .it has happened before and not saying anything it went away I'm not sure why I said anything this time...OK well he has within literally a months time already started dating someone else..mind you I know her..but he has been hooking back up with me a lot and I'm an idiot for letting this happen...so I said out of respect for her we can't do this anymore I wouldn't want that to happen to me and I was just so mad at the time that I knew her I didnt care if I hurt her....selfish I know... we have 3 kids together and I still love him.. and I know I crushed him badly the NO contact does not work in this situation but I still wanna be with him.....why? Will it ever go away