When I learned to purl, I did it backwards. For months. No one noticed, since the knitting group I was able to go to was always more about hanging out, eating cookies, discussing books, and admiring others' knitting without analyzing it (way fun, with awesome people, but it only meets during the school year. I've missed them this summer). Stockinette was always twisted. Then, as I learned more, I suddenly realized that the stockinette sections of a tank top I was knitting were all twisted stitches. So I pulled up an internet video on purling in the continental style, figured out what I was doing wrong, and adjusted my knitting.
What I didn't adjust was the way I moved the yarn back and forth for purling. I had been bringing it forward and pulling it through the loop from underneath the needle. Now I had to wrap it over the top of the needle, which required more work. Soon, to get the purls to look right, I was wrapping the yarn with my thumb. This began to cause problems. The joint at the base of my thumb would ache from the constant swirling motion. I hated ribbing and seed stitch because they took forever. I pulled up another video on continental purling and watched carefully, trying to follow the other knitter's movements. And I failed. It just didn't work. My fingers wouldn't do what I wanted them to do. I sighed, and tried to just purl more carefully.
Today, while on Ravelry, I clicked on a thread about "What I learned this week" (I can't remember the exact title, but it was in the Techniques Forum). It listed different techniques and lessons knitters/crocheters had learned over the week. I'd learnt to cable without a cable needle (another problem solved with a sock I'd been knitting--when the pair's done, I'll write about them, since they've been in the works since July), so I had something to contribute. But on this thread, I read about something called "Norwegian Purling." There was a link to a video (think that's right; if not, go to YouTube and type in 'Norwegian Purling').
This was the solution I'd been looking for. I don't have to move my yarn in front, change my hand position, anything like that. I'll have to form a new habit, but this works. It works well. And my thumb will be happy.