I don't feel very qualified to answer this as I do not know really what the answer is.
Do you still love your fiance ?? If not do you want to go ahead and spend your life with someone you do not love.?
If you do, stop messing with your own feelings, try and get your friendship with your best friend on a more comfortable footing or bin it altogether.
It is so easy to mistake love for lots of other feelings and emotions.
Ask yourself if you are bit bored with your BF, if your relationship is a bit stale. If it is and you love him do something special like a weekend away, a romantic meal in a lovely atmosphere be it at home or out.
I know this is not a great deal of help but I could not just not answer you.
I feel a bit like an agony aunt !!