I'm sorry you're feeling so badly. You are in a difficult situation.
A small note on this topic though: to some extent, you can't choose who you are, but you can choose how you behave. It seems like you may have to choose between your husband and poly (though I think anneintherain has given good advice about how to try to reconcile the two). If you must choose one or the other, realize that it is your choice based on your values and nobody can force you to do anything. Make a decision, accept responsibility for that decision and move on.
If you choose to stay with your husband in a monogamous relationship, your relationship with him still won't work if you blame him for depriving you of your boyfriend. If you choose to stay with him, you need to try thinking more along the lines of, "for the sake of my marriage, I chose to make this sacrifice. I want my husband to recognize what I am giving, but I give it of my own will knowing the consequences and believing it is best for me." If you can't find a similar sentiment in yourself, then perhaps you should make a different choice.