1. This person doesn't see your value
Imagine someone who never tells you that they appreciate you. Would you desperately try to make that person be your friend? No, you probably wouldn't. Same goes for a romantic relationship. This person hasn't acknowledged your inner beauty. Being with that person wouldn't make you happy, so f**k them.
2. There's more to life than finding "the one"
We all get taught from a really young age that romantic love is incredibly important (through parents, movies, books, you know the drill) but it's not the truth. There's so much to do and accomplish in life and there's no shame in doing that without a significant other (even if society/your family/friends/colleagues are telling you otherwise).
No, you're not going to end "forever alone", yes, there are people who love you. Your friends? Your family? Your pet? They love you. And they don't make you feel bad about it. Unlike that person you're desperately trying to get to love you.
4. Being alone is cool too
Don't feel like there is someone who loves you right now? Time to become your own love interest! Go out on a date with yourself, discover your own personality, do what makes you happy! You might even make friends during the process. (Believe me I've been there.)
5. There's plenty of fish in the sea
By far the most clichéd one but it still applies. Just start observing people: there are so many different kinds of people out there and they all found someone, despite whatever way they may look like or how they behave. It would be so extremely odd that there is someone for everyone except you. Out of all those people on planet earth.
In conclusion, if you're currently sitting in your room and are sobbing your eyes out over someone: there's so much to see and do in your life, seize that opportunity. Life is short and you don't want to spend it running after people who don't care.