@polymiami, if you get a bit of clarity in this thread, I want to say you should start a new thread, it might get you more advice, and by then you'll hopefully have clearer ideas of what you want (I always find it takes me a couple of layers of questions before I figure out just what I am upset about and want advice on)
It seems at this moment your main problem is anger...and although your head knows you shouldn't be angry at him, nevertheless you are, as you feel you signed up for one thing and ended up with another. Yes, it does sound like your partner is doing everything right, and reassuring you. It sounds like at this point there is nothing else he can do, so I hate to say some of the burden falls on you. Reread Ethical Slut, check out the Book and Website sticky in this forum. Buy Opening Up by Tristan Taormino!!! maybe you will identify some of the problems you're feeling from this so you can articulate them to him - and yourself.
Secondly. It sounds like you are spending time with him and his boyfriend. I know you like the guy but think about why you're doing it a bit - do you want to be there so you can witness what happens so you aren't left out? Do you enjoy your time as a group and it enhances your life? Do you wish you had an important second partner too and do you feel jealous, or are you even wired to love more than one person? Are you torturing yourself by spending time with him around and would you serve yourself better by being off doing something with friends, or a hobby that makes you happy instead? (maybe you're only around him a bit, but you mention it in your thread so I want to say if you are in his presence a lot, that might be useful to retreat from that for awhile while you work through this).
Lastly - if you can see a therapist and aren't, (and I think in this case I'd suggest solo therapy because I think having your partner there would shift the focus onto what he SHOULD do to help, and I get the sense this is about what you need to figure out for yourself...) get yourself there. Maybe you don't want to be in a poly relationship, maybe you will be OK with it, but pushing your partner away isn't going to get you want you want and need - not in a direct healthy manner at least.
Happiness will never come to those who fail to appreciate what they already have.