Try: I'm WAY too arrogant for you and you probably deserve someone better. I don't know you, but if you're honestly serious about this question, then I'm serious about my answer. Anyone who thinks just being good-looking makes them a worthier person to date than someone else is probably less worthy. Of course, if all you're looking for is someone as pretty as you, I'm sure you'll find her. She might not be the nicest person or the most compatible for you, but hey, she's as good-looking as you and in the end how great the two of you will look in pictures is really all that matters. As far as I'm concerned, this girl you're talking about, yeah SHE'S out of YOUR league simply because you have a complete lack of class and an over-inflated sense of your own worth because you may be good-looking.
Tell her what you just told us. Don't make up an excuse because she may think there's still hope for a chance. Tell her in the nicest way possible how you REALLY feel. When she realizes you're serious she'll get over you. Fast. Just don't be surprised if you get dirty looks from girls in her social group. Word travels fast about arrogant guys and she will warn- I mean tell other girls about your unique personality. Karma has a way of coming back so don't be surprised if you fall hard for a really hot girl who finds you not in her league if it hasn't happened already.
Are you really asking this right now? I mean, REALLY asking? You are not worth this girl's time, and she could definitely do better than you. It's not that she isn't pretty enough for you; it's that she's too good for you. You seriously need to realize what's important, and that you're kind of a jackass. If you tell her, she is better off, but for one reason: she will have spent less of her time on you. I'm sorry if I offended you, but no matter what you look like I can tell you right now that you're not half as great as you think you are, hun.