Hmm...Well the picture that I see in my head is as follows. Tell me if I'm wrong.
F-Has a girl, but is following B
B-Broke up with boyfriend so she single, but being followed like a piece of meat.
and you like B.
1. F has no business with B because he has a girl.
2. B seems to be very very pretty if she has a lot of suiters after her.
3. Yeah it's bothering you cause you a tiny bit jealous. So what to do?
hmm... Does B know that F has a girlfriend?

muahaha...
It's rough when it's a workplace thing because if you tell her how you feel and she doesn't crush back...umm... it will hurt to even go to work and see her. I don't know. I would feel as stuck as you now that I think about it. Maybe wait it out and see what happens. Just continue being her friend for now.