One-itis is a symptom of low self-esteem. You're either doubting you "have what it takes" to be with this girl or you're doubting whether you have what it takes to be WITHOUT this girl.
Which is ridiculous...you have both. You need to cultivate your self-esteem and work on being complete with yourself.
Dating other women on the side while you build your relationship with this girl gives you "options", which is the classic way to deal with these feelings. But then if your options, for whatever reason, are unavailable at the same time as your one-itis, it'll just feel even worse and make the one-itis even more profound.
Start dating women for the experience, rather than to fill a need for affection or validation. This is the ONLY way to get over one-itis. It also means, though, that your traditional media-accepted idea of "love" goes out the window. The question is...how bad do you WANT to be happy with your life?