<human>: Can you provide a brief rundown of the text in within 3 sentences?

my girlfriend [F26] and I [M25] took a break last year for about 3 months. I acknowledged all my errors leading to this break and did my best to mend the relationship. 3 months later we got back together but I always suspected she had been involved with a close friend during the break. I confronted her about it then but she told me nothing happened and accused me of not trusting her. It took me a long time to get over it but I finally accepted that nothing had happened. In the past year we grew much closer then we had been before and things seemed great. Today, a year later, I found out my suspicions from back then were true. She slept with another guy. I don't know how to handle this situation... I still love her and this happened so long ago, but it's difficult to move on knowing our strong relationship this past year was sitting on a foundation of lies. This year I went through severe issues that she has helped me overcome. She's been there for me everyday and now I feel like i have nobody. Am I wrong to feel like she cheated on me? I'm absolutely heart broken and confused. I know it has already happened and nothing can be done but should I just let it go and continue with our strong relationship (which will obviously need some repairing after this)?
<bot>: my girlfriend slept with someone else a year ago and denied my accusations so I let it go. A year later, our relationship has grown stronger than before, but I found out it did happen. What to do?