There's nothing left to do but give up.

We can't control who we fall in love with.
But, we can control what we do with that information.
If we fall in love with someone who is in a committed relationship, it's only decent of us to back off and let them and their relationship flourish. It may seem right in the moment to land up at her doorstep, tell her you love her, and ask her to come back to you- just the way it happens in the movies- but when she's been in a relationship for six months, at an age when even a one week long relationship is a big deal, it's time to realize she's happy, and that you're not the one who is making her that way. It’s time to realize that she doesn't love you back in the same way you love her, or that she sees too many reasons not to be with you. It’s time to stop believing that the very real relationship she is having is, infact, 'infatuation'. It’s time to stop telling her you’re in love with her.
If you're on your death bed, then perhaps telling the girl you love your true feelings is justifiable. But when you're in your teenage years, with your whole life ahead of you, and she hasn’t broken up with her boyfriend despite the many times you’ve told her you are in love with her, maybe it’s time to look around, and move on. Or at least, let her.
Don’t say you want her to be happy and then spend every moment of your life trying to make her feel guilty for not being in love with you.
Don’t take advantage of the fact that she doesn’t want to hurt you and that she isn’t as ruthless as you and keep telling her you’re in love with her, despite knowing it’s wrong, just hoping for a positive reply.
If you aren’t happy together even as friends, if you’ve forgotten how to handle your friendship, what is the sense in trying to make it something more?
Sometimes the boy-girl best friends end up together. They each have a string of boyfriends and girlfriends, but then suddenly they begin to feel jealous of them and look at each other in a different way. Suddenly they begin to have feelings towards each other, feelings that have nothing to do with friendship. That’s the way it happened in “Maid of Honour”, remember? But that’s usually not the way it happens in real life. Not all time, anyway. And not when the guy and the girl have stopped being best friends the moment the girl starts dating some other guy. And especially not when she falls in love with this other guy and is immeasurably happy with him, and is terrified of the idea of breaking up with him, or living without him, and when he becomes the one she goes to when she wants to complain about you, and how much pain you are causing her. That’s when you ought to realize that you have lost the battle, and that there is nothing left to do, but give up.