<human>: Can you condense the text into three sentences or less?

I (30 M)  in a 4 yr relationship with (30F). A week ago I got drunk and blacked out and a mutual (F) friend offered me a ride home. We hooked up. I think she knew what she was doing. I got half a bj and then got out of there. I remember very little. She is in our friend group so me and my gf see her often. I do not like this girl in anyway, and I have never cheated on anyone in 30 years. I have been on the receiving end. I can't decide if I should tell my girlfriend or not. I do not believe the other girl would tell anyone. What should I do?
-----------------
A lot of our friends saw me that night and everyone knows I was a drunk a**hole, completely gone. It's not an excuse. I know I screwed up big. If I lose my girlfriend it will be the biggest mistake of my life. I will tell her if I need to but at the same time I think nothing like this will ever happen again and I don't want to ruin this relationship.
<bot>: Blacked out, cheated on my long term gf. A mutual (F) friend offered me a ride home, gave me half a bj before I got out of the car and got out of there