My boyfriend has cheated on me 3 times now ...?

my boyfriend has cheated on me 3 times now, and after the second time i told him it was the last but, just couldn't leave him when it happened... and now he's been unbelievably nice, and hasn't gotten out of line since(couple months ago) and is really scared to lose me. I'm now worried that it is going to happen again... i don't know what to do.. should i leave him? and i feel like im in the wrong for leaving him if he hasn't messed up again, considering i technically forgave him for it for taking him back.. help please.