Hey there and welcome to the mind fuck. Even though you might think that our positions couldn't be more different, as I am the one in the position of your husband, I know exactly what you are talking about. I have had a really hard time coming to terms with this whole poly thing. I really hope that you won't take as much time as I needed to wrap my mind around the facts, as there have been some years that needed to pass till I was able to face the truths about my feelings and desires.
When I fell in love with my best friend, I was waiting for signs that the love I felt for my husband would decline. It was the only way I was taught love works. When I couldn't reach a point where it would have felt true to say that I love him any less and after long introspections (questioning my love for my husband, questioning the love for my friend) I had to admit, that I loved him just like before, that nothing changed and my conclusion back then was: You are just totally weird.
So as others have already said, this takes time. Keep on thinking about it and when you theoretically have grasped the concept, try to experience it if you yourself reach a point where that would be an options. In the case of my husband, experiencing it was the only way he himself was able to really decide if this was for him. He needed to feel (just as I had experienced it over the years before) that my love hasn't changed. When we finally were in that situation and nothings changed in regard to the 'feel' he got from me, he said it was ok.
If someone would have confronted me with this concept on a purely theoretical basis, I think I wouldn't have consented to 1) this being possible at all and 2) to be part of it. From my point of view, some things can only be understood when we have had the experience. It's important to build a solid foundation and walk slowly into something like poly, but you can think about it as much as you want, sometimes this won't make up for the practical experience.
To your mono/poly topic and the longitude of relationships: poly relationships are so unique, it is tricky to make general statements. In our case we will have been together for a decade this year, we never have thought about an alternative lifestyle or have had experiences outside the 'normal' monogamous marriage. And we were happy like that. When I fell in love and when I came to terms with my feelings, things changed structure wise, but our marriage stayed the same. This is still the way, we do relationships. The only part that is missing now, is the exclusivity. That's all. I think it is great that you had the chance to be together this long and make sure of what your partner is really like. You don't have to struggle like some newly formed relationships with getting to know your partner, you are used to manage the every day trifle that can become a real thread to a relationship and you have mastered some hard times already.
Don't feel insecure as long as your partner still makes sure that you have the same attention as before and don't feel insecure jut because he said that other may be important to him as well. Don't be insecure about your body and project this onto your partner. I know how hard it is when the image your mirror is showing you has nothing in common with the person you want to be, but don't judge this in your partners place, as you can't see what he sees in you. You may be just right for him.
I hope things will become a lot easier and processable for you in the near future. You have come to the right place to read and ask questions. If you haven't done it yet, try a tag search under the search options for "mono/poly" or "marriage". You will find many unique stories from others, some will be written from your perspective. Maybe that helps to think about all the possibilities out there. Good luck to you.
Facts: 30, female, bi, v-type relationship with Sward (husband, straight, mono) and Lin (boyfriend, straight, mono), poly-fi and co-primary.