I've been on the other side of this situation (at first, at least) so maybe I can offer a bit of perspective. When my wife came out as poly after 4 years of marriage, I was pretty devastated. I thought it must be my fault in some way. I thought my wife was being selfish and acting on a whim and I thought it was all about sex. I didn't want to share her with anyone. I've since learned that all these things are not true and my wife and I have an even stronger, deeper relationship than we did before, even though she has a boyfriend and I (hopefully) have a new girlfriend. The point is, your husband is probably feeling a lot of anxiety and heartache and maybe he feels like he's done something wrong to cause your feelings. You need to keep those lines of communication open. You need to tell him why you feel the way you feel. You need to talk until your brain hurts, sometimes. You don't want to lose your husband, but you're having a hard time being monogamous. I know it's really hard, and it's a risk, but maybe you should tell him how much you love him and that you don't want to leave him or lose him, but that you also need to be true to yourself. Give him time, take it slow, and always remind him that you love him and that you're not looking for a way out of the relationship and that he hasn't done anything wrong. Not sure if this helps but I hope so.
-You don't have to believe everything you think