How to get over your best friend?

First of all- we are in our 20's. I have had the same best friend since I was 15, and I have never liked him as anything more than a friend. We went to different universities, and stayed in touch and visited each other the entire four years. However, I went through a very rough patch in my life during the past year, and he has been there for me the entire time. Because of this, I have started to develop feelings for him, and I feel like I truly love him. He constantly calls me his sister and talks about how we're as close as siblings, so I know he doesn't think of me the same way. He also goes on dates with other girls. I feel so pathetic because it literally makes me cry and sick to my stomach whenever I hear of him hanging out with other girls, but I know I can't complain to him about it because I obviously have no right to do so. I know a way of getting over someone is to avoid contact with them, but this just isn't feasible in this situation. It's consuming my life, and I need a way out. I don't even notice other guys, and if I try to hang out with them, I just end up comparing everyone else to my best friend. Please help. I can't keep living like this.