My girlfriend was unfaithful, and I'm unsure about how to handle the situation.

I'm a 22-year-old guy, and it’s been a couple of months since I discovered something troubling. I initially found out that my girlfriend, who is 20, was approached by an older guy while I was at work. He asked her out, framing it as a casual lunch among friends. Naturally, I confronted her about it, and because she never agreed to the date, I decided to let it slide, but I remained vigilant. The next day, I checked her phone and found over 200 deleted text messages from this guy. Many of the messages were him flirting with her, and she seemed hesitant, but there were a few instances where she appeared interested, even mentioning, “I’ll leave him soon.” Other times, though, she insisted, “We don’t talk like that.” Before all of this, she had a significant drinking problem, which I had addressed multiple times. Remarkably, she hasn't touched alcohol for three months since I last confronted her. When I found out about the texts, I pressed her for answers. Initially, she claimed she was talking to him because I wasn’t giving her enough attention and insisted, “I don’t care about him, I want you.” When I asked her what had transpired, she described him as a nuisance who wouldn't leave her alone, so she said what he wanted to hear just to get him off her back. However, I was still skeptical, as some of her messages didn’t seem entirely one-sided. After weeks of talking about it, she finally admitted that she had gotten drunk one night and couldn’t remember what happened. She claimed he told her they had sex the next day, but she had no recollection. I know it takes a lot to get to that point, but just to clarify, she’s generally not the type to frequently go out. This man lived next door, and I had grown accustomed to her socializing outside while drinking with friends. She mentioned she went over to pet his cat and then forgot everything until she returned home wanting to shower. I don’t know if that’s a typical response for a hangover. Is it possible to lose only an hour of memory, or does that seem like a blatant lie? Doesn’t it feel a bit too convenient to lose the memory of the moment that could jeopardize our relationship? Given that she has stopped drinking, could that be a sign that she might still be a faithful partner? Has anyone experienced a situation where their partner cheated but still managed to rebuild trust in the relationship? I understand that I might not get clear answers to all my questions, but I felt the need to share my thoughts.