You good. I'm a woman and have experienced similar feelings of disappointment at finding out someone I vibe with is taken. What makes the difference is how you respond to those feelings. As long as you can continue to respect her and her boundaries, you're fine. If you need to break off the friendship, that's fine too.

The reason you're feeling guilty over your immediate response is because of all these horror stories we see (especially on Reddit) of men going off the rails when they fall "victim" to the "friend zone". Your feelings are perfectly normal.