I get infatuated too easily then things end up terribly?

It seems like I fall for a guy too easily and too quickly. I feel like I'm too trusting. I don't want to completely shut guys out, but I want to put up kind of like a wall around myself so I don't get hurt as easily. I have an amazing boyfriend, but there's another guy who I know was just playing me, but kept trying to get me to kiss him (I really didn't want to). I made the mistake of falling for him once and that's how we got into this whole mess. I think he's mad because I played along and then when he went in to kiss me I backed off. Help? I know it's really confusing but any answers will help. How do I not fall for guys who I know are crap?