GUYS: How do you know when you should just give up?

On the hope of going out with the person you like? If you have to give up then how do you fight the feeling that he is the only one for you? Have you ever felt this way about someone? Did it go away over time? I didn't think the feeling would last so long. I feel like I'm in some kind of weird limbo and I don't know how to give up and I don't know how not to give up.