Similar situation over here, was with my girl for 5 years and the sex dwindled to practically nothing. It's her own psychological issues that she's working through, nothing to do with our relationship.
I thought about cheating but realized it would end with both of us more unhappy, than if I was to just break up with her or let her know I'm going to be polyamorous now.
Long story short I eventually decided I want to be polyamorous whether or not she wants to (I brought it up and although we used to be, she didn't want to be poly again), so I talked to her about it and let her know how I felt. She said she couldn't be with me, but a few days later she changed her mind.
She decided she would rather stay with me and accept me for who I am than break up because I'm unsatisfied with certain aspects of our relationship.
I'm going to have dinner tonight with a beautiful girl who makes me feel like a king.