I cheated but he still wants me back help?

2 nights ago i was super hyper on sugar, and i know my boyfriends best friend had a crush on me, and for some stupid reason i decided to kiss him to compare his kiss to my boyfriends. (retarded right?) last night i realized what i had done and thought i better had come clean, my boyfriends best friend has a girlfriend and she was listening in when i was talking to my boyfriend, so now all four of us are depressed, shes moved away and says hes dead to her now. my boyfriend (or now ex boyfriend) is not coping, and his best friend is in awkwardness. (he kiss me back i think...i don't remember) i'm in serious depression as well. we were talking about getting married and now there's no way that will happen. over the past day i've been carving words into my arm with scissors, such as: "slut" "dirty sket" "filthy wh*re" and things that just remind me what i've done, for like punishmen,t and for some strange reason he says that he could forgive. but i told him i don't want to be with him, when he asks why i say "because i've hurt you once i don't want to hurt you again and if you make this choice i can't guarantee that i won't hurt you again everyone makes mistake but this is just ridiculous stupid and slutty and i won't let you do this to yourself you deserve better" i'm reading over this now and it seems i'm trying to gain pity, no i'm not and i give you all permission to say a couple of mean things to me in your answer i deserve any abuse you give me, but he still wants me back. and i need your advice on what to do? and his birthday is on friday what should i do for that help!?