Thank you thinker, Anne and Gala.
Your replies have helped me to put this all in perspective. I especially like the advice to "identify what [I] DO want, and present that to" my partner. That's exactly the issue I (didn't know I) was having... I wasn't getting what I wanted, and I wasn't helping the matter (for myself or for him) by casting my needs in a restrictive light.
So instead of saying "don't do x, y, and z," I'm going to focus on saying "please do a, b, and c!" Who knows? I might get all of my needs met WHILE the partner has fun with the new girl. Amazing.
Also, Gala and BoringGuy, thank you for the reassurances re: aging. I can't tell you how much I hate feeling this stereotypically female. I've never had serious concerns about getting older before. It has always seemed kind of exciting. So this irrational anxiety feels like it's coming out of the blue, and I don't love it. It helps to be reminded that I am "hardly anywhere CLOSE to any of those [ages]. DECADES in the future. Hang in there!"
It also helps to interact with awesome ladies in the forums who aren't my age or younger. Because now that I think about it, most of the women in my social circle are 30 or below. It's scary being the one who has to figure out her 30s before everyone else gets there. Maybe it's time for me to branch out, huh?