Pulling away from the guy I was head over heels for?

Ugh, this always seems to happen to me. I love the chase in the relationship but once I get him, I just start picking out all his flaws that will drive me away! Why do I do this? This guy is perfect for me, but I can't help but roll my eyes at the stupid things he keeps saying. How do I stop looking at the negative and think of the positive? He's getting so clingy! How do I tell him to slow it down without causing a fight? Thanks, please help!!