Girl, I feel you. I had the same problem for the 4 years I was in college with my best guy friend. He never had a girlfriend when I liked him, but he also made it clear that I was un-dateable...so I get it. After college, I ended up moving to the same city as him so we still see each other on weekends, but I've been focusing on my new job and I've also met a new guy that makes me feel good, like a crush should. That helped me realize that I was wasting my time feeling bad about something that wasn't going to happen.
The best advice I can give you is to try to meet more people when you have free time. Try to minimize the amount of time you spend/chat with him. You'll see each other at work, so maybe after work you can do something by yourself or with other friends, to try to meet a new guy or get involved in something that will help take your mind off him. I think the key is finding something that makes you feel good and can help you let go of those bad feelings. I hope this helps, good luck!