Well there's a difference between telling yourself someone was no good and actually believing what you're saying to yourself. Lets use an analogy here to illustrate that point. Lets say you're at a fair and you're starving, so you go to this booth that has the best pictures of food you like and the price is right. Well you order that food to satisfy that hunger. But after trying it, you realize that not only was it not as good as you thought, but you end up getting a really bad stomach ache that puts you on the toilet for a half hour. When that clears up, you're now not only weak from that experience, but you're still weak from not eating good. Are you going back to that same booth because the pictures still look good? Probably not, because emotionally you're no longer tied to the picture of the food, but what eating it did to you. Chances are good you will find a different booth and look closer at the food itself and not rely on pictures.
Now lets use that analogy with this girl. You have this great picture of her in your mind, but what you got was not what you imagined it to be. The picture was better than the reality of the person. So why are you still hung up on the picture? Well its because emotionally you're still trying to reconcile that the experience you received did not add up to the picture you have in your mind. In other words, emotionally you don't want to believe what your mind is telling you, probably because the answers do not give you the satisfaction the picture did. So you really have two courses of action here. You can apply this analogy and see if reason and emotion finally shake hands on why the experience is not worth going through again. Or you can seek her out once more and reexperience that belly ache and possibly get something much worse that finally aligns your emotions with what you mind is telling you. I don't believe its anymore complex than that. Experience is a great teacher! And what I believe is true for all situations is that we don't always fully register the answers to our problems until we have emotionally had enough. Sometimes that requires more than one trip to the booth. I hope this helps.