I'm pretty new to poly and I came to it more as a very sudden solution. Maybe you can glean something from my experience.
In my past, I've been abused- sexually and mentally. This has made me shy away from males and lean more towards women. Even though I was married and totally commited to Andulvar, we would have sex with other women (one was Marius, whom I'm dating now). However, I couldn't make that leap to an actual relationship....I couldn't get past myself. We established one rule: if it impacted our relationship, we don't do it.
After talking a lot to Andulvar and Marius (as well as my 6 other lovers) I just leapt into it. Many people would say that this was a bad decision however, we all went very slow and Andulvar was and still is very tolerant with my limits. I came to find that this feel natural to me and I don't think I could go back to being mono.
As I read more of your posts, I'm not sure if your husband will go this slow with you. Of course you know him much better than I do but I urge you to take a hard look at him. The bottom line is, if YOU do not want to be poly then YOU do not have to. Do not let him make you feel shitty just because you're not comfortable.
I hope this helps somewhat and best of luck.