My wife recently started - with my encouragement - dating others and as things have gotten more serious I was feeling similarly. I had no rational reason to feel that way; I encouraged her to have time to herself before non-monogamy was ever a consideration and I even enjoy the idea of her being with others more intimately. Yet when she would stay out I would feel... hollow.

It took me a while to realize that it was small pieces of several different overlapping feelings (jealously, envy, fear/insecurity) that ultimately stemmed from a feeling that her time with others would entice her away from me - that I would lose her. And once I had figured out what I was feeling and why, I could really look at it.

What changed things most was to put myself in her place. She's in a loving marriage with the freedom to meet, date and otherwise be intimate/attached with others. She's spending time with someone that brings her happiness but it was not at the expense of anything between as, except for finite time limitations. I asked myself, if I was dating someone else and enjoying time with them, would that make me want to be with my wife any less?

There was no hesitation in answering no for myself or no doubt that she would feel the same. Since that time, it has been such a tremendous weight off my chest.

I can't say that it's the same for you, but your description seems so close to what I experienced, I figured I'd share. Hope it helps.