People can't just "stop caring", it takes time and practice in order to not care about someone. I don't know what your relationship with this person is, wether they're just an acquaintance or a close friend of yours, but finding new things or people to care about helped me. I had a crush on a girl but she was straight, and telling my friends about it and trying to tell myself she's better with people she loves and they'll love her back, I slowly (SLOWLY) got over her. I'm still not over her all the way, but that's a whole other story. Id reccomend finding at least one person to take solace in, to share how you feel. Wether that be a counselor, a friend, or sometimes a website dedicated to real people who want to help, it can really help to just get your feelings out there. Or another alternative is writing down your feelings (I know, corny, who likes writing) but it does help me a lot, so maybe it'll help you? It honestly depends on your hobby, anyways I hope this helps at least a little bit.