Hello, ill start with telling you a bit about myself and my girlfriend.

We are 22 years old and have been together for 5 years, she is my first love. (we were both virgins, if that matters).

I love her extremely much and we spend most of the day and every night together.
I am in a pretty challenging university and I took some exams around 16-24 may. Last two months before the exams I was barely at home any time of the day. Basically i only slept there, was home late every night.
I found out on 20. May that she had been talking with this a bit older guy from work on Facebook and through texting, it started at the end of April. (a lot of texts and Facebook messages).

I confronted her and she was really sad and told me that I was never at home. she meant that she wanted/needed good attention. I asked her if anything happened and she told me that he kissed her once at work, that is all that happened.

I asked her what they talked about and she told me that he is married and they talked a lot about their relationships and he has 3 kids and wants a divorce but is to afraid. He complemented her a lot, which she said that she liked.
She stopped talking to this guy(he had started another job at the beginning of may) when i found out.

After that we talked a lot and we want this to work. She thought that I had stopped loving her or something, she wasn't in first place in my life anymore. I corrected that.

Today I wanted to go through her phone records, just to see when this all started. She didn't want me to but was with me when i did and didn't stop me. There I found the texts (I btw never saw any texts or his phone number in her phone), from there I know when it started exactly.

The last month we have been doing very good, we love each other very much but I have doubts..

What should I do?