Ow. I'm really sorry you're hurting. This is real, you are not at fault for feeling this way, and I strongly doubt that anyone on this board would ignore your pain.
You appear to be monogamous. That's simply a way to be. Monogamy can be so completely part of a person's identity and self-understanding that it is who they are.
Polyamory is also a way some people are. It's not better or more enlightened, it's just a part of some people's identity and self-understanding. Your BF may be poly. Let's assume for the moment that he is.
In that case you have one of the fundamental relationship problems: you're a mono in love with a poly.
I think it is possible to learn to live happily in that situation. But it takes time and work. I do NOT think it is reasonable for you to expect yourself to simply accept his poly nature and move on. Everything in your self-identity is saying NO-NO-NO-NO! and you can't ignore that.
I wish that he recognizes your immediate and real pain and steps up to help you. He's going to have to hold you very close and make your relationship to him his first priority -- he's got to make your emotional self understand how much he needs and values you.
I hope that you can, over time and with care and self-understanding, gradually accept that although you are his center and his pillar, he also needs another love(s) to feel himself whole.
You may never understand completely how that feels -- and you shouldn't ever beat yourself up for not understanding completely. To take a trivial parallel, it would be like someone who loves Mozart not understanding how anyone can obsess over baseball...those are just two different ways of being. But I think it's possible to accept and come to peace with it even if you don't understand at an emotional level.
But. You need to make a choice, and you need to make it clearly and as unemotionally as you can: you will either begin the long process of accepting how he is, or you will tell him that the poly relationship is not possible for you. If you choose to try to accept his polyamory, then he needs to help you right now. If he is unwilling to compromise and help you, that's a red flag.
WARNING: everything I've written is based on one internet post and my very feeble sense of how these things work. It's possible I am very, very wrong about a lot of things! So take it all with a healthy dose of skepticism. You are the only one who really knows because you are the only one who is right there in your shoes!
Good luck! Please write more; I know that there are many people on this board who will sympathize.
Addendum: ksandra posted while I was typing. I agree with what she wrote. Her post is very wise about love being a renewable resource, and VERY right about talking to your partner.
Last edited by EugenePoet; 05-16-2010 at 05:35 AM.