What you aren't understanding is that you won't be able to control your feelings in the way he wants you to. You aren't going to be able to put him first in the way he needs when you love two people. This isn't because polyamory doesn't work, because you can't do poly or because he doesn't understand polyamory. It's because he needs relationships that aren't necessarily monogamous but are couple focused in a way that inherently restricts emotional availability outside the dyad. Which, alas, is something you, like many of us, seek.
So, what you have to do if you don't want to break his boundary is completely avoid forming relationships where you can start to build those emotions. That means you keeping any outside relationships to casual, NSA, swinging type encounters. That's the only way you will be able to keep his boundaries and not treat someone else badly when your partner pulls a veto again.