One thing I repeat a lot is that once you end a relationship, truly end it. It is easy to say this in retrospect but seeing him was a big mistake. You were only giving him false hope. Tell him in no uncertain terms that you two are finished and he is never to contact you again. Also, nothing is ever meant to be. You are the master of your own fate. Apologize to your current boyfriend and ask him to help enforce your new no-contact rule, with force if necessary.