I had this situation when I was 25 and already working, still lived with my college buddy as a roommate.
This drop-dead gorgeous girl moves in upstairs. I'm talking Brooklyn Decker in real life, so we're both h**** and interested. I tell everyone at work and talk about the girl every day.
One day, she invites me up to watch late night TV/movies. Out of nowhere, she says she would like to give me a massage, don't worry, she does this for all her friends (can't make this up, it still makes me smile to this day). So we're watching TV and I notice she's sitting on me, massaging my back. I couldn't help but feel something is going on.
This is a Thu. night, and as luck would have it, I was going to visit my folks in CT for the weekend, and was leaving Fri. night. Told her that, and next thing you know we are kissing, this was unbelievable!
When I came back on Sunday, she was waiting, and you can guess what happened. Man it makes me smile as I type.
Next day at work, I guess I was out of character because I didn't talk about her. I get back in my office, and this dude Dennis taped a paper on my monitor (it was a CRT back then lol), "STUD!"
Now to the point--my roommate was so jealous, he took a day off from work, and made a pass at her, telling her we were close friends, like brothers (we were), and so we share our women. WHAT???!!!!
Needless to say, we had a falling out that lasted 3 years.
Put it this way, my friend was in my wedding party, so you'll usually sort these things out, but when it comes to women, nobody likes 2nd place, so to the OP, you have to go for it, or you'll never live it down until the next one.
I'm married now with a son, but somewhere in my attic are pics of that girl. Haven't looked at them in years, but it's just a nice memory of when my g/f was like a SI swimsuit model. Incredible.