Lonely significant other (M53)

After 14 years together, I feel so alone as a partner in a gay relationship. We have a wonderful life, but the connection between us has slowly dissolved over the years. Our sex life isn't what it was - although it never was 3 times a week thing. Now, it's once every three weeks. I feel very depressed and alone. Where did my best friend go? Where is my soul mate? I'm 53, so I'm looking at the rest of my life and wonder if this what to expect? I want to make the most of my life, not feel like I'm watching it slip away.