Should I Confess or Bury it Away?

I have been in a very good relationship for over 2 years now. I love my girlfriend very much. I would never want to hurt her. Well the other day I had some friends over for a party. One of my friends has a crush on me but she knows I will never leave my girlfriend for her. But the other night we got really drunk and found ourselves alone. Long story short she ended up wanting to see me spank the monkey while she had her breasts out. I guess at the time we felt this wasn't cheating. And I thought it was kind of exciting having her watch me knowing she can't actually have me. But the next day when I sobered up I realized how wrong this was and now I feel absolutely horrible. It makes me feel so bad that I realized I would never want to do anything even remotely close to cheating on my girlfriend ever again. But I can't seem to let this go. She most likely won't find out because this other girl also feels horrible and admitted we should never do anything like that again. I've read many similar forums that say it is selfish to tell your significant other and hurt her over something that I know I will never do again. Plus ever since I did this it made me realize how much greater my girlfriend is than I ever considered. And I have vowed to always treat her extremely well from now on. And she has been very happy since I made this change. It also shocked me so bad that I have even almost completely quit drinking. I have always drank too much and this made me realize I want to be a better person. What should I do? Confess to my embarrassing and selfish act in order to be completely truthful and relieve some guilt, but at the same time crush my girlfriend and our relationship? Or bury this secret deep down, sparing her that pain, and swear to be a better boyfriend to her? Please no judgement, just honest help. Tweet