You're not alone
My husband and I have been together for over 17 years and we have two small children. A few months ago, through a series of events, I came to realize that I am poly. For years, I felt something missing, but I never knew what. Now I know. I brought it up to my husband and his initial reaction was much like yours: confusion, fear, hurt, anger. However, through LOTS of discussion, reading posts on this site (both of us are here on the site), and marriage counseling, we have come to an agreement that I am allowed to explore a relationship outside of our marriage. He is firm in his belief that he is mono, so we are going to try to make this mono/poly relationship work. It's been the most stressful 2 months of our lives and we have been on the brink of divorce a few times, but so far, we are hanging in there. Some days he's at peace with the idea, and days like today, he's resentful, jealous and hurt and is sure that I want an outside relationship because he's "not enough" for me.
I think the biggest hurdle for him has been coming to understand how I could love him AND someone else at the same time. I struggle with guilt because I don't want to hurt him and I feel responsible for turning our lives upside down, but at the same time, it's a relief that I have finally "come out" with my true feelings. It's been a struggle, and will likely continue to be for a while, but I do have confidence that we WILL get through this together - and that our marriage will survive. We love each other madly, and cannot imagine lives without one another. It's worth the effort to try to work this out - for us and for our kids.
Being new to this, the best advice that I could give is to read as much as you can about polyamory with an open mind. Our society has taught us that monomogy is the ONLY way, and this is the barrier we must break through. More than 50% of marriages end in divorce, many times due to infidelity. Why break up a family if there is another way?
My husband and I are desperately trying to negotiate a compromise where we can keep our marriage (and family) together, and get both of our needs met. For me, this means an outside relationship with LOTS of limits (at first anyway). For him, this means that I give him tons of reassurance with affection, sex, and telling him the many reasons why I love him over and over again. It takes a big effort on both of our parts, but we get a little bit better every day.
If you and your husband still truly love each other, it's worth it to explore this lifestyle. If divorce is your only other option, you've got nothing to lose at this point.
Please know that you are not alone and that there is a way to make this work if you both truly want it.