So this situation has been on my mind for quite a while now and I've finally decided to turn to random internet strangers for help.
Last September I (a 21 year old male) transferred to a new university and joined a student organization there (the details of what this group does aren't relevant). I met a girl about the same age as me and took an instant liking to her. We would chat during our weekly group meetings when we got the chance, but before I could ask her out she mentions having a boyfriend.
This was obviously a disappointment, especially since I only seem to develop romantic attraction for women very rarely. In any case I thought it would still be worth us getting to know each other so I asked her if she wanted to meet up on campus sometime to get coffee or whatever. She seems enthusiastic about this and we make plans.
Time passes and our little meetings continue (maybe every other week) and my feelings for her grow stronger. We talk about her boyfriend a bit, and I learn that they've been dating for just over a year (by now it'll have been year and a half), and that they were friends for at least another year before that. I even ask if they're close, and she says they are and that her boyfriend is nice, although she doesn't sound terribly enthusiastic.
As summer approaches, our group's meetings come to an end and we both plan on returning to our respective hometowns (she was living on campus), meaning we won't have as many opportunities to see each other. I message her suggesting we try to keep in touch during the break and she agrees, but when I try to set something up later she doesn't respond.
Taking this as a final sign that she isn't interested, I decide that I should probably stop tormenting myself and I unfriend her on Facebook as a way of putting distance between us.
Our group did have an unexpected meeting in the summer (about a month and a half ago now) and I saw her there. I wasn't planning on snubbing or ignoring her in person, and once we have a few drinks (it was a pub night) the initial awkwardness is defused and we're laughing and joking with each other.
I haven't contacted her since then, since I still think I should just try to get over her and find a single woman, but I can't stop thinking about her. Worse, I can't even seem to think of other women in a romantic way because of my feelings for her, and I have had other opportunities that I've ignored out of disinterest.
So what I'm asking from you, dear reader, is both advice on how to move on from a situation like this (if that indeed is the correct course of action), and also insight into the situation I've described above. If you want more details, have questions or need clarification just ask and I will provide. Thanks a bunch.