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Sorry, LW, but looks like it's over. And not because you had a threesome; your revelation just quickened the dying of this particular relationship. Living in a conservative and fairly religious state, I'm fairly certain that most guys I know would respond as your BF did. It takes a long, long time for social mores to make it here, and that sort of experimentation just wouldn't fly yet. I guess we're in the baby bird stages of getting past sexual double standards. So, I don't think your BF is nuts, just awfully judgmental. You're the same person you were when he met you, and you can't undo anything in your past no matter how badly you or he wants you to. For whatever reason, he's hung up on this and he's not willing to get over it. Time for you to move on.
That being said, I think you've overestimated this relationship for a while. You claim that because of it, you are happier and more confident, yet in you also say that you've been worried from the start that he would find out something about your past that would cause him to reject you. If you can't be who you are with your partner, what kind of life would you be building together? Certainly not one that you would find fulfilling in the long run.
Perhaps give him one more chance. Tell him you love him, but he either has to find the maturity to get over your sexual history or you're leaving. Quit groveling and apologizing and do it now. Don't wait. You can always find a new apartment and get yourself a kitten or two for Christmas.