How do you deal with feelings of loss when a partner starts dating again, even as an experienced poly person? & how to cope when your partner is much more “successful” at dating?

Background: my partner and I have both been poly since before we started dating, and have been together for 4 years. Since then we’ve had other relationships, but nothing else lasted. We don’t do prescriptive hierarchy, but descriptively, our lives are pretty enmeshed (we live together and share some finances). In the last 2.5 years or so, neither of us had any other relationships. My partner wasn’t dating at all & I went on a few dates that didn’t go anywhere.

A couple months ago, my partner went on their first date in years, hit it off with the person, and now they’re dating. It’s hard not to see it as all downside for me. Of course there’s the upside of them being happy and finding love, and I love that for them! I’m not wishing them ill or hoping it’ll end. But it still feels like a loss for me—of time, and of money because they are spending mutual money on dates & airbnbs, and just, like, a loss of peace and stability. For me, it just feels like a big disruptive change, while for them, it’s all NRE rainbows.

My personal values around poly are that I’m not going to get in the way of their relationship, and if I need to set a boundary, I can remove myself from the situation. Eg, new partner wants to stay over at our house, I’m not comfortable hearing them having sex in the other room, so I’ll go sleep at a friend’s house.

This has worked fine so far, but I find myself dealing with these really deep feelings of loss, worry that my comfortable life is going to change for the worse, and feeling like my abandonment wound is getting activated.

It doesn’t help that I haven’t had many successful dates in the last few years—I guess I’m not super interested in anyone right now, although I’d love to experience the rush of a crush again if someone came along. I feel (irrationally) frustrated that my partner so easily found someone. And then I feel like a jerk for thinking that.

How do y’all deal with these feelings, so many years into polyamory—if anyone can relate?