It's simple - this is a friendship. Your colleage likes you, but she has a boyfriend, so she sees you as a friend at work, and she's made that obvious.
What you do now is up to you. If you are content to have her as a friend, carry on, but drop the telling-her-you-like-her part - she will have got that already, women are intuitive about these things.
If it is going to upset you to want a relationship you are not going to have, then cool it a little, be nice and polite, but keep the contact down to a working relationship.
A word of caution - don't let your feelings affect your work, and your relationships with other colleagues who havea lready noticed your attention to this woman. Work is just that - work, and your bosses will take a dim view of your love life becoming a subject of gossip - gossip equals distraction equals loss of work, and you will be seen as the culprit.
Add to that the fact that relationships with colleagues are difficult for all sorts of reasons, and can end badly, and you still have to see each other at work.
On balance, I'd put this down to experience. Keep this lady as a work friend, and colleague, nothing more, and set your love sights on someone outside work - it will be better for everyone, especially you.