Ok, this is a difficult and delicate position to be in, however, I am not going to tell you to give up just yet, not at least until you ask yourself a few questions and answer them honestly.
What are your chances in conquering such love?
Does he or her know that you exist? Has he or her gave you any hope in having his or her love back?
Do you believe that, if continue to pursuing such love, what are the chances that it will hurt you more than bring you happiness? Is it even worth to continue suffering or is it time to move on?
How long have you been chasing it?
I hope that you if you make a self analyze about your current situation, you will be the best person to determine whether this lost love is something worth fighting for or no, only you can determine that.
If you truly believe that is worth, then, by no means, try everything that you to conquer it back, otherwise, move on.
There's no such thing as that he or she is or will be the only love of your life. Allow yourself to be happy, if he or she doesn't even know about your existence, then, do yourself a favor, move one with your life and, treat yourself in a manner that you deserve it.