When you start seeing someone new, start investing time and energy into your "relationship," and devote yourself to them, it can be a little upsetting when you come to find out that it was all for nothing. After all of the effort you put in, you realize it never meant half as much to the other person as it did to you. It's unfortunate, it's painful, but it happens.
I spent the fall semester trying to convince myself that I just wanted something simple, something casual. I eventually found myself in a "relationship" with someone who was constantly tossing me back and forth. One minute they were supposedly thinking about dating, the next they wanted nothing to do with it. Eventually I just couldn't take it anymore and a big "fuck-off" was in order.
As it turns out, casual really isn't all that simple after all. Once you start realizing you have feelings for them you have to come to terms with the fact that it's about to get a lot more complicated. You also have to consider the fact that the feelings might not be mutual. Or if you're as unlucky as I am you find yourself in a situation where the feelings are mutual, but your priorities aren't. And once your feelings are known and your relationship leaves the realm of "casual" there really is no going back.
I speak from experience (no surprise there) when I say that no matter how many times outside observers tell you that you're wasting your time or that you'll only end up hurt - 9 times out of 10 you still won't listen. I am a firm believer in the "enough is enough" rule. But the key to the rule is realizing that everyone's versions of "enough" are different. Some people can keep themselves on an emotional roller-coaster for months, others can't survive days. In my opinion, I'd much rather deal with the pain that comes along with "enough" rather than moving on too soon and meeting enough's best friend "what if." It is up to you to decide which you'd rather live with.
But eventually you owe it to yourself to say I'm done, I deserve better. You have to be willing to wake up from your fantasy and realize that someone who can’t give you what you’re willing to give them is not worth your time. And this is easier said than done in almost all circumstances, but eventually you'll find your threshold, draw your line in the sand, and learn how to keep your standards where they belong.