This is just an outsider's perspective, but it sounds like he is in a hard place. You've described the relationship structure as having been, for a long time, that they were primaries, with a secondary relationship between you and him. That can be a stable long-term structure.
You've decided that you don't want to be secondary anymore, and so he's trying to make adjustments to keep you from leaving. She doesn't want the structure to adjust. She might even be concerned that your desire to shift from secondary to co-primary could also manifest, down the road, as a desire to shift from co-primary to one-and-only.
It also occurs to me that if anyone in my relationship structure asked me to make a choice, between them and one of my other partners, I might be inclined to choose the one who wasn't making me choose.
You ask whether it's selfish of you to make the decision that you don't want to be secondary, and I don't think that's important. You have to take care of yourself, and if living in a poly-fi secondary relationship is not meeting your needs, you have every right to want to change things.
Has it been poly-fi until now? I think it would be difficult to be secondary-only in a poly-fi relationship, but that's because I have a lot of needs that have to get met. I can do that if I have a number of secondary relationships, but not just one.
If I were in your shoes, I would probably resolve to gently end the relationship and move forward, in hopes of finding something that is more fulfilling and with fewer landmines. I wish you luck.