Wow, talk about timing. I am in an almost complete opposite situation as you, and I still fucked up! I just finished typing a mope-y post in the GAF relationship thread.
Here's the abridged version.
I met this girl hanging out with another guy in a new city. I didn't think much of her, but she was pretty. Still, we did hit it off, and I needed more friends in a new city, so I gladly accepted. We got talking, and became good friends. We invited each other out often. I knew from day 1 she was single, but I didn't care. I wasn't into her. We continued to hang out for the past few months, and I've even tried to find a guy for her.
After hanging out for so many months, I realized I developed feelings for her. Shit hits the fan, because after I realized I had feelings for her, she tells me this guy she's had a crush on also likes her back. This like literally happened a week after I figured out I liked her. The thing is, this guy is from Scotland, so he's in another country.
After a lot of thinking and internal struggle, I said fuck it, and decided to let her know I have feelings for her. To my surprise, she said she's thought about us being together as well. Except now with this guy coming back into the picture, she doesn't know what to do. I know for a fact she likes this guy more than me. Now it looks like the guy is flying back for her, and I'm SOL. So now all I can do is move on, and probably cut her off, even if just for a while.
I lost a girl that was single, and was into me. I got you beat, OP!