Russ, this woman is bad news! The most important things you mentioned in your letter were your age and the quote "the most amazing person I have ever met." Those things are very related - you're VERY young, and you're going to meet much nicer and more amazing people in your life. Sorry to be so harsh, but I'm looking in from the outside, remember, and my judgment isn't being clouded by my infatuation with this woman. In fact, your own judgement isn't completely gone, either - you said yourself that you might be blinded by love or devotion. I've been where you are, and it's wonderful to have these feelings for someone. But, dude! She's sleeping with another guy, and the best thing for you to do is tell her it's over between you two. No qualifications, no conditions (as in, "it's over unless you stop sleeping with what's-his-name"), no regrets. And if she still wants to come out here to see you, make it clear to her (and to yourself) that you'll only be hanging out as friends. If she has someplace to stay that isn't with you, all the better.
You've got tons of loving and learning to do in your life, but don't let anyone walk all over you and toy with your emotions.
Good luck and enjoy your youth :-)