Need advice and help

I got back together with my ex and I really shouldn't have made this decision. I recently found out that he's slept with other girls he met at a mixer and online and he belongs to a group that gathers together to "socialize". He doesn't know I know. In fact, he doesn't know that I know where he's going tonight. He lied. Yes I went through his stuff and found this out. But I wonder if I was accidentally given access to this information for a reason. But to make matters worse, I cosigned a car for him. Yes, I know I'm dumb. I don't think he's going to purposely mess up my credit, but I'm afraid to confront him. He's going to deny everything unless I show him proof. But I don't want to tell him I've been going through his stuff either. What do I do? Is this one of those times where someone says "well you've made your bed now lie in it"? I'm so sad. My heart is feeling so broken. Why do people do this kind of stuff to each other? We even had a conversation about being truthful to each other. He flat out lied to me about where he's going to be tonight.