You make good points, all around. Your reply serves as a good reality check for me, and a reminder for me to keep up the communication with my partner.
I just wanted to restate that, while the age difference between my partner and the new girl might squick some people out, we are still talking about consenting adults. I'd hate for my issue to get recast as a "creepy older guy is taking advantage of immature females" story.
For the record, my partner has dated women who were his age, and older, in the past. And he's not laying any sort of propriety claim on the new girl. She's absolutely free to see other people. The two of them have entered into a relationship that allows the new girl her to explore her kinks and sexuality with a more experienced partner, for a while. The partner and I both believe in the campsite rule ("Do no harm, and leave it better than you found it."), esp. when it comes to less experienced partners.
I guess I'm saying that I don't actually "feel weird about her being 21 and [him] being 44 and the 23 year age gap." I'm just dealing with irrational insecurities of my own, as I compare myself to something that I used to be (a college girl, an honor student, a precocious, bouncier, more naive, more explorative, less experienced human being).
I know that I am currently an awesome lady, whose mental, emotional, and intellectual experience makes me irreplaceable and attractive. But that doesn't shut down the nagging little voice that says "You just aren't as CUTE as you used to be." Cue the louder voice that says "Who wants to be cute, anyway? How much better it is to be capable! Rational is sexy!" Trust me, though. I could go in that particular circle for hours.
Anyway, that last part has more to do with weird personal insecurities that I hadn't been aware of until the new relationship started to bloom.
Which means that my final question is, actually, how the crap does one deal with learning new, unpleasant facts about oneself? I thought I was secure, open and free of jealousy. Talk about shattering my self-image. I'm disheartened to find that all it took to bring my insecurities to the surface was the seed of a new relationship.