lets make something clear, no one was FORCED to have sex here. You might say you were too sleepy to 'think' but hey..thats what you tell yourself to make you feel less guilty. Just the way you started your post, how this girl is pretty has has spent nights at your place, its pretty clear the desire was not new on your part...and when the opportuntiy came, you did what you wanted, stopped thinking. I agree with Pink, and find you and this girl at fault. Now step up and face it like a grown up!! I think you should tell your girlfriend about what happened. And leave it up to her to decide what she wants to do with you and her friend. If I were in her shoes, I would probably beat up both of you until I run out of energy, then kick both of you out of the house. I dont know how your girlfriend is, so be prepared for all kinda reaction from her side. I think what you did is pretty unexcusable, but hiding it from your gf and continue life like 'nothing happened' is even more pathetic. You need to take responsibility for what happened. You might lose your girlfriend, but she is much better off without you. You have broken a trust for lust. I doubt this Resha girl takes you in high regard so I dont see your future with her either. Sorry for coming across harsh, but you asked for opinion, and this is what you get.