I have to second dragonsky, that part confused me as well.
I have taken the challenge of a new love as well after being in a relationship with my husband 10+ years. I have to say, that obviously each relationship is unique and has it's own pace, but I wouldn't compare them or say that one will always be the more deep and important one for me. It's like degrading my feelings for my boyfriend. Love is love, why would I want to dimmish it just because it hasn't lasted as long as the other relationship has? The older relationship has naturally developed different and more things that are important to me, just because it has been around longer, but that doesn't affect the fact that I simply love them for who they are.
I know that many don't do the co-primary thing, but you have explicitly agreed to try it and saying that you can't picture her in an equal position next to you is kind of nullifying this concept right from the start, leaving her in a really awkward position to be in. Maybe some of her insecurities and problems stem from the unconscious vibe you are sending while keeping this negative attitude in mind.
My husband has expressed the need for being primary (in a primary/secondary arrangement) in terms of his coping with the thought that my 'new' love (actually some years old at that point in time, but never mind) would degrade the importance he bears to me. It didn't take him long to see how unhealthy that wish was for us and especially for me. Because he tried to dictate how I should feel, should act on my feelings and how the relationship of my boyfriend and I has to be. This highly unfair attitude was off the table after some weeks (could have been days even, he was really fast back then). The main factor that caused this uncertainty was the new situation, in which he wasn't exclusive with me any longer. Meaning for him: he had to let go of his sense of ownership of my emotions and me in general. This never meant in return that I was less committed to him. Or that my feelings for him changed in the slightest in a negative direction (the opposite was true, I loved him even more for the work he did and the deepened connection we were able to establish).
You have been in this for quite some time and I wonder, if this was a problem from the start that hasn't been addressed or if it just came up after polyfitri lost her husband and appears now as a greater threat to your relationship with your husband than before. IF some of this sounds somehow right, maybe you could talk to your husband first and try to figure out, why you aren't able to feel secure in your relationship. Because that's what it came down to, when my husband has had those problems. He was easily assured after those weeks that I love him like before and still rests assured till today that we are on a healthy way and nothing in regard to our feelings for each other has changed for the worse by me having another love in my life.
I hope you are able to find some peace of mind soon.
Facts: 30, female, bi, v-type relationship with Sward (husband, straight, mono) and Lin (boyfriend, straight, mono), poly-fi and co-primary.