Doenitz,
I know it hurts to not only lose someone you love. But not being able to meet the other person freely because she is now attached would hurt me even more, putting myself in your shoes. But I feel if she really takes your friendship seriously, she would meet you anyhow. Can simply keep it from her boyfriend right? Although it might not be politically correct but that's the way to keep the friendship going.
Perhaps she was feeling lonely when she asked you out or something. Enjoy your break from this girl, you will definitely feel better and someone new and interesting might come your way, who knows?
Take care of yourself and all the best.