I am in the same type of situation!
But my best friend is married to a guy so she is off limits no matter what!
I am close enough to her that she is more of a sister to me then anyting else and I have said "i love you" but I always phrased it to sound like it was a general "your part of my family" sort of sentiment then the "i think your really hot and want you as a girl friend" sort of sentiment.
I am also really good friends with her husband so I just couldn't do anything to disrupt either of those friendships.
After a few years I did realize that I was attracted to women that look a lot like her... I even quit a job because I couldn't stop lusting after my manager! When I thought about it later I realized that she was almost a carbon copy of my friend!!
It is complicated but I decided that I wanted her friendship more then a relationship.
I am now contented with what I have! Two really good friends!