I used to feel this about my relationship with my bf C, that it 'wasn't going anywhere'. In retrospect, I think what I felt was that we were not on the same page even though we kept telling each other, that we were.
Despite his frequent assertions that he was definitely poly, I think I always felt that he wasn't. He wanted to be, but when he got another gf besides me, it became clear pretty quickly, that he could not be in 2 intimate relationships at the same time. She was single, and mono, and lived near him. I was married, with other partners, lived 2 hours away. No wonder he drifted away.. he is now monogamous with her.
When I started dating again I was very cautious. At first I thought I should only date men who really identify as poly.Then I thought but that's what C did, and look what happened! No guarantees, ever.
So now I'm dating a guy who has zero experience with poly, and he's open to it, and we'll see where that goes.
early forties, straight.