"You do have the right to say that poly is not something you want.
You have the right to ask that things slow down.
You have the right to expect that boundaries be respected and to follow through on the consequences if they are not."
Quoted for truth.
I am sorry you have found yourself in such a difficult situation, desire. It's quite understandable that it would be very upsetting for you. I think your partner (husband?) got caught up in the NRE thing, and just let things spin out of control. Now he has two women in his life, and he is responsible for being fair and honest toward both of them. Maybe that is not easy for him, but he kind of got himself in this mess. He needs to step up and do the right thing, by both of you.
Don't let yourself be spooked or guilt-tripped out of asking for what you need in this relationship. You shouldn't have to feel forced into something you're not ready for.
Regards and sympathies,