I don't enter relationships that I can tell are one sided like this, and when I do, I'll leave them immediately.
Yes this happens all the time, but for me its pre-relationship, I like a girl a lot, she doesn't return my same feelings or at the very least is on the fence. I walk away from these situations. Some girls have been tough to play hard to get and think I'll keep hacking away until she gives in and decides to date me. Sorry, but I have better things to do with my time than chase a girl who doesn't really want to be with me, in hopes that I can change her mind, which is very unlikely and as other have said, will likely be a one sided and unhealthy relationship to begin with.