cheated on me

So me and my bf were dating for 2 years and then things started going down the hill for me so i guess i wasnt treating him the best and i kinda was with him only because he said he loved me so so sooo much and would kill him if i hurt him , so i stayed trying to see if my love can get stronger rather then fade it didnt i messed it up and broke up with him and went for another guy (yes i know im a jerk and an asshole) so after 3 months my ex and i started to talk again and we saw we still care about eachother and now we are dating again for 8 months not counting the past 2 years and everything is amazing and we are better then we were in our last try .BUT about a week ago he tells me that hmm he cheated on me so here i am hurt beyond belief only because he was the one who use to tell me how he would never cheat on even the worst people he dated but yet he did to me .. after telling me he would die before hurting me . and then he told me he did it because i treated him bad ... but i dont get it why not just leave me =/ i know i made my mistakes but i atleast broke up with him and never cheated on him ... LET ALONE have sex with some one shit kissing isnt that bad ... but SEX? i guess im really hurt because here i am thinking he wouldn't hurt me and i use to think he was better then any guy i can find and wouldn't cheat on me . i forgave him but everytime i look at him that image pops up into my head and i asked him why he did it and how and he said it just happened but how does sex just happen after kissing some one i would stop and think hey i think im doing something wrong here . he tells me he didnt enjoy it and didnt even do shit the girl did since he was spaced out and upset at me . i have really hard time believing that .

i know i wasnt the best gf but i dont think i even deserved that .
and yes it was our past relationship and a big mess .. but still! it really hurts to know some one i trusted 100% hurt me when im the one who always messes up im just shocked .

sorry if this is a mess im bad at writing and this is just random thoughts and feelings that i cant even begin to put together .

run away. he wants it all. he's selfish. he went back on his word. you cant trust him. there are others out there.
this is just going to tear your relationship apart because there is no trust. and if hes done it once, he'll do it again.