I sincerely wish I could help you definitively (I'll still try). This is not impossible, because I've read exactly that this can be done and how precisely this is done: how to stop loving a person, lose all feelings and never look back. But I suck at finding words, especially in the forum format.
You need to convince yourself this person is not good fit for you, then it should stop. This is, of course, no easy task, but it is the real destination. What you might read about limerence, etc. is a distraction. You are most likely both good people, but two good people may not necessarily belong together, even if it were reciprocal. Hence the clue is: reach the conclusion that you two absolutely wouldn't belong together, that a relationship would not be good for you even they also pursued it. Don't build an argument about them having flaws, for your judgment of them wouldn't be stable anyway until the love is over at least, and to make it over is to convince yourself that specifically relationship would be to your detriment.
Support yourself through the quest in whatever way you can: remember what you were capable of, even if it was long time ago. Console yourself if need be, prepare that you might sometimes lose self-control and impulsively make effort to contact them - don't let that ruin your morale (although prefer not to engage them still, of course). You're not lesser for having developed love for somebody who doesn't love you back - you are through a difficult quest, most likely alone, which means low opinion some others might potentially have is not legitimate, especially if they don't come to help or only do so in condescending way (by this I don't mean any one from here, but rather those who invented the narrative which frames person's struggles as being their fault, their failure to adjust the way others seemingly do).
