Learn to cherish your friendship with that person. Sometimes, a friendship between two people is a better fit than a romance that may end up in a bitter break up or divorce.

You move on. There is no value in maintaining that friendship. Plenty other women out there.

Find another but doesn't mean can't be friends. I tease a few married women at my work almost elementary or high school style. Just having some fun with flirting with them but since they married and most with kids the only thing developed is a playful friendship. 

Also usually when you are getting some or have gf then girls somehow sense it or more confidence in you so they want some of that confidence or pheromones.  Get another girl and maybe current friend might want you more. 

Move on, especially if its a work relationship. Trust me, ive been in more than enough those to know. Stay friendly just in case situation changes.

Ask her to be friends with benefits?

Try to bring a little bit of distance between you and her, hang out with girls outside of your workspace, if she really likes you as a friend she will understand and once you're more easy with the situation you can go back and be just friends with her if you still want to.

I once had a very similar situation in 11th grade and now 6 years later she is one of the few people from school I'm still friends with.

Move on.

Drop her and tell her you don't need more friends.

Be friends...?

Become an Incel.