McUZI, I don't know how old you are, but don't let your hormones do your thinking for you- if you are afraid of losing whatever level of friendship you might have with this lady, I suggest you go look for another, as you won't be comfortable with this one you have your heart set on- who is, as you have said, presently involved with somebody else. Don't get too worked up about it or her. If you both are young, say, in your twenties, this will all eventually pass and be replaced by new concerns and interests. (I have been exactly where you are...)
Besides, if she would dump somebody to be with you, you too, could be the dumpEE at some point.
Women are like buses; if you miss one, another will come along, sooner or later. (Yes, ARlady, the same thing could be said about men, too, I guess.. :^) ) Live life, don't take it too seriously, and go shooting- takes the mind off things...