Should I leave him forever this time?

Hi, well iv been with bf over a year now.about 8 months into relationship i find out hes been going on dating sites looking for 'fun' with other people,and lieing to me over and over again on so many levels.since then, iv been trying to forget what he did,along with many other betrayals since then.he actually ended things with me 2 months ago because he couldn't cope with the way i still was about everything, and i found out at this point that hed been secretly emailing a girl trying to arrange a date(on his dads computer) whilst acting as if he still loved me and would never risk hurting me again.he came crawling back after a week saying hed made a huge mistake and that he wanted me no matter what. i,being the stupid person i am at times,agreed to try again cos i love him in spite of everything.the thing is, since then, my curiousity has gotten the better of me on more than afew occasions and iv gone online to see if he has profiles on other sites.he does.such as findmea*****buddy.com, a secret msn address/profile on which he was calling himself single and 'looking for fun' and others.i know the only way he could still be visiting these sites is if he was going on his dads computer(his dad does similar stuff to his mum!!!!), and iv told him that i need to know that hes not going online full stop as it seems hes just too tempted to go back to these sites.i just couldnt trust him not to yet.i just dont think this is a healthy relationship anymore but im torn because i cant imagine life without him!for the record, hes 26,and i dont think someone of this age who has been in committed relationships before should be doing these things to someone they love.i just cant get past the fact that he wants/wanted a '***** buddy'-i'm not like that at all and he assured me for a long time when we met that he wasn't either.wev always had great sex,regularly,its not boring by any means-so i can't see why he would be like this!so sorry for the long rant but i'm so torn right now and i could'nt sleep last night thinking about it.