I haven't read any other posts besides the OP.
It feels good, doesn't it? Feeling so connected to somebody else feels so good. She makes you feel good inside. And it doesn't hurt that she's pretty hot. It makes you forget her shortcomings. It makes you feel good that even though she's with somebody else, she's probably in love with or is going to end up with you. And in all honesty, you'll probably never get over her.
And its going to hurt like hell when you leave her alone. Take my advice (or don't). Be her friend or leave her alone. If its more than physical and you've let your heart take over, no good will come from this.