Having suffered from one-itis for a span of 4 years ( yes , Its possible, girls are fucking cruel sometimes) long story that doesn't need to be told at this point.
The best way Ive found of actually getting rid of one-itis entirely. Sarging like you've never sarged before. I'm sure its been mentioned, but go out and fuck twelve girls, and then see how specially she really is. If she's still special at the end of it all... there's a simple solution, rinse and repeat. Another twelve never hurt anyone
Having a bud to go out with whose got your back also helps out, try find yourself a wing man who helps reflect a better you. If you can work well together you should be able to line up an easy twelve.