Why can't I fall in love?

I'm a very level-headed person, always have been. I've never fallen head over heels in love, but also never had cold feet when it came to the opposite sex. However, I've had three relationships, and all of them have ended with me losing my feelings for the other person and ending it before I hurt them. I'm really not insensitive, and I truly care about all three of these guys. I'm back to being close friends with all of them, just as I was before we were together. I want to experience love so badly, but every time I try it falls apart. Now, I've met a guy who is completely different from all the others, as cliche as that may sound. He just gives me a whole new perspective on things, and compliments me without being all gushy or being all over me, which are my two biggest pet peeves in a relationship. I don't wanna lose my feelings for him. I want this to work and last a very long time. Am I just not attaching to a guy the right way, or am I shutting myself down, or what? I try so hard to be a perfect girlfriend, but I never seem to follow through. I'm willing to share any more information that anyone needs to know, I really need some help.