Hey man, I'll try to reply best I can.
First off, I think you're just another victim of the modern image of "the man". You're one of those guys (like me) who does not feel comfortable being a club-goer with the Game as your bible. First thing you have to do is realize this. Sorry if I'm making any assumptions but you remind me of myself. Accept and embrace the fact that your qualities isn't a six-pack and overdose of self-confidence - it's other things (arguably more important things.)
Sadly, guys like us shine when already in relationships, not when commencing one. Which is why we don't get that many one-night stands, and might end up feeling like losers.
I'm certain you're a good guy. And it's a shame that you've fallen in love with an occupied chick. But whatever you do, don't give up. You gotta keep hope alive, just don't let it contain you. Whoever says you should give it up, sorry for saying this, does not know passionate love. It sounds cheesy but I know exactly how you feel! I gave up on two girls I felt that way about because of this exact reason - them having boyfriends. The first one even said I was the love of her life, but stupid me didn't have the balls to steal her from her boyfriend still.
And you always end up regretting it.
So keep being nice to her (not pushy) and eventually the crazy love will tone down. If it doesn't, or if she breaks up with her boyfriend, tell her all of this! Don't just say "uhm.. I like you.. kinda", tell her the whole truth.
Dude, just because you're 26 and a virgin doesn't mean you're some sort of idiot or freak. You just probably haven't gotten the opportunity yet. But now, with a girl this close, you have. Congratulations.