Help! I’m attracted to my girlfriends best friend?

Mike Asked: Help! I’m attracted to my girlfriends best friend?

i guess i should start from the very beginning. me and my girlfriend madison have been sharing an apartment for about two years. four months ago her best friend sam wanted to move in because she was having money problems with student loans of 100k+, and we decided to be nice and let her stay over. sam was really nice and liked video games and skateboarding which are stuff i liked but madison didnt. sam was a really laid back kinda girl, while madison was more uptight and controlling. plus sam was smoking hot (madison is average). yesterday when madison was out for work i was watching tv with sam. sam started to cuddle saying she was cold. then i put my arms around her. i wasnt thinking straight and soon we were having sex. afterwards she said promise me you wont tell madison about our relationship or she will kick me out. i promised. that night i couldnt sleep cause i felt so guilty. i wanted to break up with madison and date sam, but then she would really be pissed off and i dont know what will happen whether she will kick sam out or not. i cant look madison in the eye now cause i feel so guilty. what should i do?