I'm sorry NotPolly. The poly lifestyle is really not for everyone. And it can be crushing when you love someone who is polyamorous knowing you can never be. But you probably did the best thing for the both of you. He is free to love as he is meant to and so are you. Neither of you would have been happy remaining in a relationship clouded with jealousy and pain. Please try to take comfort in the fact that being unable to accept his poly lifestyle is not wrong on your part if you are truly a mono person. I must commend him though on being honest from the beginning. None of this means he loved you less or you loved him less. Some people are just not compatible. Good luck to you in moving on and finding the love you are searching for.
If you want to share more of your story please introduce yourself in a new thread in the "Introductions" section. You'll get advice and support there, I'm sure.
I'd rather be hated for who I am than loved for who I am not.