<human>: At the beginning of July, I broke up with my girlfriend of 3 years. I was heading to college and didn't see us going very far. The past 2 months, we have been talking again and got back together a couple weekends ago.

Before we got back together, though, I found out that she went to a party, got black out drunk, and made out with a guy. I thought about it and realised that it wasn't that big of a deal. 

Recently, one of her friends confronted me and told me that this her and this guy that she "made out with" actually had been fucking for a little bit. They started a week after we broke up. 

So what should I do? How should I feel? Obviously I'm pissed and upset. I know that the chances of me finding a girl who hasn't been with other people is very slim, but the fact that she slept with someone so shortly after a 3 year relationship is really bad, to me anyways. On the other hand, we were single, and she's free to do what she wants.

How will things be back t the way they were? I talked to her about it almost as soon as I found out. She lied to me, multiple times, about only making out with this guy and was upset that I found out about it. But she was also upset what she did. I asked her "why so soon?" and she said that she missed me and he gave her attention. But a couple days after I broke up with her, we started talking again and hanging out and we went to a concert and I had been telling her how much I fucked up and how much I needed her. So she knew how I felt. She said that she couldn't trust me yet, but I still don't understand how she could get with another guy. We were each other's firsts for 3 years straight. I need some advice

Summarize the given text in a few sentences.
<bot>: Ended a 3 year relationship, a week later she fucks a guy a couple times - now we're back together and I'm finding this out