I just slept with my ex.. help!!

I broke up with him one week ago after a year and a half relationship. We even moved across country together. The root of the problem of our relationship is trust. The straw that broke the camel’s back was that he had a second fb account linked to his app and messenger app, but claims he doesn’t know how it got there. Logically, this makes absolutely no sense and I know he is lying which is hurtful. My insecure self jumps to the conclusion that he’s using this extra account to cheat. But if he had nothing to hide, why have a second account in the first place?

Anyways, in all of my past relationships after we break up, I go cold turkey and never speak to them again-cut all ties. This time it’s different. He wants to still be with me, he said even if it’s just friendship, that I mean too much to him. Within this week I know he has been speaking with other girls on dating websites and hitting up classmates. I said this to him tonight, and he claims it’s because he’s lonely and that he only loves me. In a moment of weakness, we had sex. I wanted to. I want everything to be back to normal and to be able to trust him. Idk if I can. Has anyone been able to regain that trust with an ex? How do you even regain trust? Am I just hurting myself in the long run by keeping my relationship with him? I feel like from the outside looking in I should be yelling at myself “what are you doing?! He doesn’t love you from what he has done!” But I don’t want to be right