Let's be honest, you kind of are doing something wrong (see answer to MikeyG for more info). You are flirting and hoping for more of a relationship. So, you have three options.

1. Continue to suppress your feelings, see each other in only safe, social situations, be miserable for a few more months, and eventually decide to break off the friendship so you can get over it.
   

2. This is the right choice.

   Tell her how you feel, tell her what you would like to happen (you should probably think this through ahead of time) and give her the opportunity to react - possibly in your favor.
   
3. Respect her boyfriend's wishes and stop seeing her, and work on moving on with your life right now.

You should choose one of the above options based on your tolerance for risk and turmoil.

#2 poses the most risk with the greatest chance for return.

#1 will draw out your turmoil the longest but has virtually no risk. #3 has some risk - i.e., you may lose your friendship, but also minimizes the long-term turmoil.

I'm always an advocate for honesty(#2), but in this case you have to think of her feelings, too. Is she happy with her boyfriend? Would you be ruining a "good thing" for her if you made her question her relationship with you and her boyfriend?

Good luck deciding.

- Judith