<human>: My girlfriend was back in her hometown for the weekend and I attended an old school friends house party/small drink up. The night was going good until I found a girl there was someone who my SO hates from their high school days. I don't have anything against her so acted friendly. She started getting very physical with cuddling up to me on couches and massaging me whenever she had the chance, harmless. I still tried to keep my distance until I had consumed a bit too much alcohol and returned arms around her for a short time. People then suddenly started going to bed and we ended up on one couch for a little while. I didn't kiss the girl But she, or we, got a bit handsy and did a low amount of down pants. I then went to sleep and she wasn't there when I woke up. I  got her number off the friend I went with and told her I was sorry and so on. She replied things like "People do silly things when they're drunk", "Don't worry. It never happened". Her and my SO have mutual friends so it could possibly get out that way. But i'm really unsure of whether I should tell my girlfriend as it's not fair to her. Or try to go about life as if it never happened. 

Some more reasons I might not want to tell my SO. She is quite emotional. On a few occasions in our 11 1/2months. she has physically harmed herself over silly things. E.g I didn't have sex with her for about 5 days and kept falling asleep at about 11pm. She deducted that I don't find her attractive anymore and cut her rib with a razor blade? Pretty scary to think what she will do now if we break up. I also love her.
I KNOW I'm a bad guy. Scum of the earth. Don't deserve her love, attention or anything else. But I don't want to break up with her.

Summarize the given text in a few sentences.
<bot>: Went to party. SO's enemy and myself put hands down each others pants and performed minor sexual acts while down there. I am now a cheater and don't know if I should tell SO for reasons stated above.