Do you think because we met on a dating website and I know he's used them when weve been?

I think you should stop torturing yourself with wanting to know the answer to that question! You've split up now, and he's never going to confess to you if he was using the dating websites before you broke up. It sounds as though you've got a reasonable amount of suspicion about it though, and in my experience there's no smoke without fire. But really, WHY is it so important for you to find this information out? What is it going to change about anything? You're obviously upset that you guys have broken up, dwelling on this aspect of your past relationship will not help. Try and fill your time with something a bit more positive instead. Good luck!