A note? Good grief. Katt, you tried to talk to her. Personally, I think writing a note is a cop out. You're both supposedly adults. Adults, especially people that are supposed to be friends, talk through these situations. You can only apologize so many times, Katt.

You didn't intentionally hurt her. You didn't go behind her back. If the person was off limits she should have stated it waaay before now. You are not a mind reader, nor should you be expected to be a mind reader. Don't beat yourself up about it too much. This has no bearing on your ability to be a good friend.

She'll get over it. It would be one thing if you "stole her boyfriend". But you didn't. Men will come and go. It seems like this "crush" is enough for her to end a friendship over. Hopefully, she has weighed her decision carefully. If it were me, I would give her some space.