Of Course you probably already thought of this but its really what you need to do.Think about what kind of person the boy who started liking you again is like....if he seems more right for you then go for it, but chances are if hes already left you once, hes not the right guy for you.
If the guy your with now seems to be totally in to you and you like him more, and hes a great guy, then you need to stay with him.
Sometimes its really hard to know when if its the right time to break it off with someone, especially if their totally into you.