Me and my bf always arguinggg!!!?

me n my boyfriend have been together 2 n half years nw. an in july he left me an i had wid sumone else so did he. he later came bk to me an wanted me bk an everythin, so we got bk together thinkin we cud sort things out an be together like before. but then ever since that happend we argue at least once a week about me sleeping wiv sumone else he calls it "cheating on him" but it wasnt bcuz we wer seperated but anyways yh we always argue bout it, but he makes it out as if im the bad person in it all because i did it first !! he did the same thing too on the same day i did so what does it matter we wer both in the wrong lol. so i keep tellin im this. but he doesnt have any of it, he says if i had come bk to him that day he wouldnt have ever gone an slept wiv sumone else. its really pissin me off cuz im made to look like its all my fault, ive forgot bout what he did an nt jealous or anythin bout it cuz she was really ugly lol, but he just cnt seem to get over me duin what i did :S .. i am trying really hard to make things work with my bf cz i really do love him and just want him to get over it, how can i make him see sense lol and make him get over it an stop bringing it up all the time???