Thank you all so, so much for the thoughtful replies. I have asked myself if I would have been as upset if he had been, say, texting his mom, and the answer is probably not. It's almost entirely about the other girl. He's been ambivalent about getting a ring from the start, so I haven't had hard feelings about his low interest in the ring buying. It was the other girl potentially knowing we were ring shopping that bothered me. All the stories have in common my feeling like I don't want her thinking she's more important than me. I wondered if this was a feeling other people have had. It's that I don't want her thinking she's special to him, to be completely honest. And this is me digging into the darkest, most embarrassing parts of myself to admit this.
I've accepted the fact that I can be crazy jealous, but this is the first time it has ever caused me to dislike the girlfriend. It normally comes out with me being angry with the bf, not with the girl he's interested in. I wasn't sure if anyone else had the other interest as the object of their jealousy.
I also appreciate the support from people who acknowledge there isn't one right way to do this. I've detached almost entirely from my local poly community because I got tired of being lectured about how I'm doing it wrong because I don't cheer every time my bf has sex with someone else. It's taken me years to finally accept that I have jealousy, and I feel like it's been a positive evolution to the place I'm in now, where I can accept the jealousy as a challenge to work on and stop judging myself so harshly.