Break up with him in a very nice manner, but be firm and precise. Take time to get yourself together, do not jump into another relationship. Just remember that when you are really in love and you feel about someone how he feels about you, what would you like the other person to say. It can happen, especially if you create negative Karma by being mean when you break up with him and not giving him time before he sees you with another person, common curtesy. Well lets just say I like Karma, it works both ways, babe.