How do you forgive yourself?

I was a month into talking with this one guy. Things were getting pretty serious. This other guy showed interest in me and i went out with him and he kissed me. I made it aware to him that i was talking to someone else but he didn’t care since I’m still single. For a split second I did like him or just cared for him because he is a nice guy. I don’t know why I went out with him and i completely regret it. I told the guy I was talking to what happened. He was hurt but still wants to work things out. I just can’t help but feel really guilty. I don’t know why I did this considering I have been strong on my loyalty. I understand what I did was a mistake but does anyone have any advice on how to forgive yourself? I feel like if it don’t fully let the guilt go then it can be hard to mend the relationship or what was to become a relationship back to how it was.