Any advice for my situation?

So my ex and I broke up mutually in December. We stayed in contact occasionally but within the past couple months have hung out somewhat often. He repeatedly said he wasn't seeing anyone or hooking up with anyone and I believed him at first. However after that I saw on twitter something about him having sex and then when I was at his house I saw a used condom in the trash. I confronted him about it after and he admitted he had sex but said it was a one time thing. I don't so much care about that as I do the lying. I hate being lied to. I told him he either had to show me hes trying hard to get me back or to leave me alone completely. He said he wants to work on getting back together but I dont know if I can trust him. What should I do? I'm 20, he's 22.