felll in love with a best friend

Never, as long as she's already committed to someone else. If she felt that way about you, she would have waited for you to make a move. If you say anything about your feelings now, it will make being together uncomfortable and your relationship with her will never be the same. I would try to be around her a little less because if she ends up staying with this other person, she will not be able to spend much time with you anyway. If, in the future, she should end her other relationship, be ready to come back in and let her know how you feel---that is, if you haven't already met someone else in the meantime.

of course I will tell her. We should put minimal secret to our best friend. Personally I believe in best friend than in my girlfriend. I know you feel it strange, but at least listen to my reason. Best friend mostly know who you really are, and they don't mind about your attitude, they don't lie to you, if they don't like anything about you they will tell you with nothing to hide, and mostly you can take those critics. They care so much about you. While girlfriend or boyfriend may look special in front of you but you don't know who he/she really is. Because they afraid if we know there's something bad in them, we might step away. You'll find out who really they are after you married him/her. And most of them ended up in divorce. I rather pick my bestfriend as my girlfriend, and if she's not I rather consider my girlfriend as my bestfriend.

How long have you been friends?