Juliana, it sounds like you're feeling a loss because what you had before has changed, and you weren't prepared for it. Maybe you're wishing that life could go back how it was, and mourning over the realization that this wish will not come true.
Your polyship has moved from one where you are in more of a "primary" role to more of an "equality" role. That in itself isn't so bad. After all, you are married and so your non-marital partner has always been in a less-focused role than your spouse. Perhaps you can empathize with him wanting to have a partner in his life who is available for a more full-time type relationship.
It seems as though the biggest hurdle is overcoming your expectation that the situation was never going to change, and now it has. As mere humans, we have a tendency to get attached to certain arrangements and outcomes. While we're well within our rights to fight for those outcomes, usually it's more productive to change our expectations to be more in line with the facts.
If you've been polyamorous for 20 years, then you've probably been in relationships where you were not their sun and stars. From now on, this relationship will be just like those. The only difference is that this time, you recall another arrangement with him. You may even feel the need to mourn the loss of this arrangement, as it was clearly very precious to you. Going forwards, you're likely to find more happiness in the new arrangement by letting go of the attachment you have to the "primary" role and remembering that he can share his love with other partners without taking love away from you.
“As I am sure any cat owner will be able to tell you,
someone else putting you in a box is entirely different
from getting into a box yourself.” —bisexualbaker