I don't mean as much to him as he does to me. That has become evident. That is my heartbreak.
A different perspective (it might not be pertinent, but whoever knows what words might be meaningful to another?):
I don't believe that GG means as much to me as I do to him. (SHEESH, I know I'm getting serious when I start writing real names and have to edit!)
That has been true from day one, 20 years ago. In the beginning, I pushed him away, because I felt it was the "right thing to do" since I knew he wasn't ever going to be my "one and only" or even my "primary only," or whatever. (I really lacked terminology then and it's only marginally better now.)
Eventually, I gave in to having more than friendship with him, but spent many years trying to make use of our FWB status, to eek out details of what he wanted in a full-time woman, so I could try to find that perfect lady for him. That failed also.
Finally, I accepted that he is who he is. He's stood up to all I could put in his way, including having me full time and then losing me suddenly to whoever tickled my full-time fancy. He has chosen to make his life about me. I haven't chosen to return the favor. He is a PART of my life, a meaningful part, but he isn't the most important part of my life and never will be.
Sometimes, I imagine that must be devastating for him, although he very rarely says anything to confirm that for me. But, I know it is OFTEN devastating for me, the knowing that I am not willing to give him that much of myself and knowing that someone else probably would, because I do love him and I would happily support him having someone else, if he would do it.
It's painful to acknowledge and accept that the level of meaning and importance differ between two lovers. But, part of the strength and confidence in our relationship is that we do acknowledge and accept that great difference. Much like what PN wrote to you (quoted by someone else above) we share a dance. Often, I choose to dance with other partners, but always, he remains a part of the dance.
It's an odd thing, when we all started talking, Maca felt that Mono was so much like GG and I DEFINITELY see why. Of course I also see the very obvious similarities he has with Maca. But the telling thing for me in all of this, watching your life being lived, your stories develop, is that from day one, I felt he was so much more similar to me.
Maca thought I was nuts. But, the further the story goes, the more it seems to be true. Even though the most basic "easy for the world to see" details seemed to be similar to Maca or GG, when he wrote, he mirrored so many of my thoughts and beliefs. And even now, his actions and choices seem to mirror my internal workings.
I don't believe this will "fix" anything. But sometimes a person WANTS to be something, but they accept that they aren't ever going to be, for any number of reasons including life experience.
I don't think Mono CAN be what you have now decided you wish he would be for you. I don't think it's for lack of care or love. Just as I care and love GG. It's because he isn't able to put together enough of the puzzle of himself to guarantee that he can do right by you that way, and he won't take that risk. Not for you, not for anyone.
If I am right, I totally know how he feels. And even reading about your heartbreak and pain, which I have devoted myself to doing, I understand his distance. Because I have held myself aloof and distant similarly when GG tried to press me for a more significant commitment.