Mack, I was you from about six, seven years ago until now. I would venture to guess you are either shy, very picky with females, or looking for something committed too early and too quickly. I was all of the above myself, literally from my middle schools years until the last straw for me recently (now going into junior year of college...) And it even took me quite a bit to comprehend this recently: one of the main reasons for developing oneitis is limiting your options. (This is from a thread around here.) Also, it is worse if you are shy and slow to make a move. I have not read this here, but learned the hard way: the more time you invest in some girl, the more unwilling you will be to relinquish her, hence your oneitis will worsen. So in essence, it is a product of being shy and/or picky TWO fold.
You have the option to turn it around earlier than I did. I mean, I've heard of guys several years older than me even, turning it around, but I will tell you, trying to break the mentality and social habits in college is much harder than in high school...admittedly I'm not at all far along yet.
OP: "If you had it with someone else then this current girl can't be all that special because oneitis is fungible." Very true, this is often what I try and tell myself. I will explain though, how a oneitis prone mind can spin it: Well, I got over this other girl, because this new one is even better. However, this old one was AMAZING, so I can't risk anything to lose this new one. (Pedestal) I speak from experience, and have at last learned the error of my ways. It simply has to be disregarded as erroneous logic.