Conflicted feelings

I'm to be married to the man I've been with for 4 years. I love him and can't imagine my life without him. But for the past couple of months feelings toward my best friend have started developing on both sides. At the same time my fiancé and I just haven't been connecting the way we used to (that started a bit before my best friend and I started to have feelings grow) as well as arguing on a consistent basis. I could really use some advice right now, I'm really conflicted.