Wow, that's tough. So sorry that you are in this position. It really sounds, though, like you did everything right, and through it have learned a bunch about yourself.
Don't know if this makes you feel any better, but if Gregor is monogamous (which it sounds like) and you are definitely poly (which it sounds like), then if you were to just be with him, you wouldn't be happy, because the poly thing would be tugging at you.
It's a crying shame, I know, but just based on the poly/mono thing, if Gregor isn't willing to accept you for who you are, then it's just not going to work out, anyway. You have a basic "bottom-line" or "non-negotiable" that just means that you are not compatible.
Please check out The Birdcage - an open, friendly Polyamory forum for all parts of New York State
"Listen, or your tongue will make you deaf." - Native American Proverb