... he is out mtg new people, having firsts all over again, making new memories while I am left out.
This sounds like you are describing "new relationship energy," NRE. If it's only been 3 years since your h started practicing polyamory, with one new partner, I am sure he had that infatuation stage big time for at least the first 2 years with her. Infatuation with her, and infatuation with finally practicing polyamory. If being poly was a long time desire of his, kept under wraps to protect your feelings and the marriage stability, now that he's actually doing it instead of just wanting it, it must be an incredible rush.
HOWEVER, that doesn't mean your feelings and needs don't matter! It's not right for you to just be left behind, doing mundane housework, taking the kids to their activities (how old are they?), sitting home in the evenings watching TV, while hubby is out wining and dining his gf, going on romantic dates... Heck, one time my ex-h came home from a date with his new gf in our early awkward days of being poly and said they'd gone to a bookstore and he was reading poetry out loud to her!!! When was the last time he'd read poetry to me?
Now, your h and his gf have broken up, despite his desire for 2 primaries, 2 households. He is eager to get back into the saddle of looking for the next Ms Right? And you're like, oh shit, here we go again.
Put on the brakes! Ask him to take a break from dating. Work on your own relationship for a while, set a time frame, 3 months, whatever YOU feel you need. Ask for what you want. More dates? More firsts? More memory making?
I asked for that from my ex, eventually. We started going on more dates. Weekend trips. Adventures to other states, towns, scenery. No, he didn't read me poetry (heh) but we went to music fests, we got hotel rooms, and as a result of all the new stimulation and time alone, we had tons and tons of sex, more than we'd ever had in our lives together (20+ years together at that point).
And/or maybe you want him to finish that household project. Or go to couples counseling with you. Or keep the kids while you go visit your sister for a week. Or tell you he loves you more. Or make you breakfast, or scrub the kitchen floor. YOU decide what makes you feel valued and loved.
(For me, all this wasn't enough. For reasons other than poly, we did break up, we'd grown apart, and it was just time.)
It is easy for 2 poly people to understand and 2 poly people are also gtg attention elsewhere and gtg to experience new things and firsts all over again too. But if you are mono, yes you get your partner but you also have to share them and struggle through jealousy, insecurity, etc. with no outlet. He came out as poly to me almost 3 years ago and it still is not easy for me. But I do want to be with him and that's why I am still here.
You now have an outlet to talk about your jealousy and the fact that you've put your own needs aside for so long. You're here and venting. Once you feel you've vented enough (you determine how much you need to vent), Examine this: "What are my needs?" Do you even know?
Not, I need him to not be him, to suddenly become mono. But, I love him, he has to be himself, how do I get MY needs met for feeling loved and secure and entertained while he continues to be true to his poly self?
Once you feel your needs are fully met (some needs met by him, some needs met by yourself, some needs met by your platonic friends and family), you will be more able to develop compersion (happy to see him happy) out of the ashes of the jealousy you used to feel.
You just need to look at what your needs are, and go about getting them met. Backburnering your own needs for decades is not healthy. I know the Christian culture has told us to put our own needs last, but personally, I do not feel that is healthy. Everyone's feelings and needs matter, not just the need of your h to be poly.