What should I do here?

It's been a really roller coaster year in terms of my happiness.

I'm in a tough situation because this girl and I, whom I really liked, were talking for a while but it didn't work out. It ended on a bad note and we don't speak much anymore. This was about 2 months ago. I though I had gotten over her, but I find her constantly in my thoughts. I know that I can't date her, but I still like her. In addition to this, I'm trying to rekindle an old flame I had with a girl last year, but I don't see it happening. There's also a third girl that I like whom I work with. I see her all the time, and we're great friends, but she's talking to someone.

I feel like my happiness is very unstable and that I could fall into a depression again. Can anyone give me some advice on what I can do? I'm 17 if that helps.