Another Nay
I gotta start by saying that you are one lucky guy--I've been through this sort of crap 3 times and I won't be twenty-two for another few months. Also been in and out of therapy for the last three years as a result...
But, in the hopes of saving the insurance companies some money on psychiatric bills, I've even come out of the lurk-void for you (haven't posted here since the Eric days, as Taliesin

)
Now, I mentioned that I've been in this situation 3 times--twice as the "Morgan", and once, most recently in your shoes. #s 2 and 3 sucked. In #2, my long-term girlfriend hooked up with my roommate/ex-friend on the sly. Think 'retributive strike'. Sure, everybody was in love, blah blah blah--net effect--nobody talks to anyone else, psychological scars all around, and after a year, I sorta feel better (and am quite glad to be single.)
In situation #3, I was in your shoes, and I told "Morgan" and "Ann" both, thinking honesty was the best policy. The only reason that this wasn't a f-up equivalent to #2 is because they were only casually dating, and things never really went anywhere with me and the "Ann." I'm still good friends with both, thankfully, but like I said, it was EXTREMELY light stuff--the L-word was never mentioned on ant front.
I saved #1 for last, because although it came first chronologically, I only found out about it recently. A very good, very old friend of mine (that I happened to game with) told me a few months ago (while drunk) that he'd been in love with a girl that I dated several years ago. He never mentioned this before, to either of us, although in hind sight it made a lot of sense. Your situation sounds like a carbon copy of this one, and I urge you to follow my friend's example. Why, you ask? Because--WE ARE STILL FRIENDS. End of story. The girl, as usual, came and went--each of us still keeps in touch with her, to a small extent, but that's pretty irrelevant. The main thing is that our friendship is still intact, which it probably wouldn't have been otherwise. And he, of course, has a S.O. and is one of the most content a$$holes that I know
I know that it can be tough, but try not to focus on the 'immediate reward' aspect of the situation. Focus on the friendships, and all the things that you are able to share with both of these people right now, rather than a probably mythical romantic future with this girl. I t may sound trite, and it took me a long, painful time to figure out, but women will not make you happy. And if they say they want a dog, they really mean that they want a kid
Amos