Never ever ever ever ever ever ever ever ever EVER tell a girl how you feel about her.
You should always get a girl's number after first contact, make it a habit.
Secondly, whilst I approve of you going after the girl, despite already being in a relationship (it's fair game as long as they are not getting married or are married) you should ask yourself a few questions before attempting it. Is the boyfriend a [censored]? Do you intend on keeping her as your girlfriend if you succeed, and not as a toy for a couple of nights? Are you willing to provide more than her current boyfriend is providing? You say that she is going away soon, that pretty much doesn't allow you to have a relationship with her so I wouldn't attempt this.
On the other hand, you have a shot at having a fling with her. I do not approve of this however. Girls in relationships are the easiest to have a fling with cause most of the time their boyfriends are so boring or inefficient that they get desperate. The longer the relationship, the easier it gets.
If she wasn't going away, I would have given you the green light. It may be immoral, but fair game is fair game as long as they're not getting married as I said. A girl is happiest with the strongest.
I also recommend seeing other girls yourself, you sound heavily deprived of some lovin', which is why you're nearly obsessed with this girl in the first place.
I appreciate your comments...
Just to clear one thing up though...I'm not the sort of person who's into flings. For reasons I can't really talk about (read: religious), I'm not looking for someone to just hook up with - one night stands, flings, casual six etc, really not my thing. I'm all about the long term. I wouldn't even think about wanting to be with someone if it wasn't for something serious.
I realise that I barely know her, and I'm not one of those people who instantly falls in love whenever anyone sans testicles starts talking to me! The things that did it for me was the last time I felt that kind of instant chemistry with somebody was the girl I wound up spending 3 years of my life with...kinda confused me a little, to tell the truth!
EDIT: Also, I'm not deprived of lovin'...I'm not the sort of person that bases my self worth on whether or not I'm in a relationship. It really doesn't bother me...I go out, I meet girls, I get on with them...purely platonic, because random, ahem, 'encounters' don't really interest me. Every now and then I'll meet a girl that I think something might work with. We'll go on a couple of dates, I'll realise she's not the girl for me, or we want different things in life, we go our separate ways. I'm happy the way things are! But I just wasn't sure whether I should do something about this. General consensus is no, basically confirming what I already thought

Another also...I did get her number, I tend to make it a habit too!