I Think My Husband Has A Side Chick! What Can I Do?

Dear readers am confused and losing it. I have been married for 3yrs now to the best man. Before marriage, sex with him was great...then I looked forward to every moment with him. Few years down the lane my husband seems to have lost interest so bad that we can stay two months without sex. Recently I noticed he couldn't even get an erection with me. I suspect there is a side chick and I really don’t know what to do.