I suffered with wrist pain for years due to heavy computer usage and nothing was helping. Wrist braces were crap and going "keyboard only" failed with too many websites, games, and apps. Sure, I could use less of the computer and save my wrists that way, but gosh, self control is hard and computers are such fun. The closest thing I had to success was using my left hand to control the mouse instead of my right, but that led to something even better...
One on the left, one on the right; one for each hand. Both control the same cursor. Switch between the two whenever you feel like it.
I've done it for over three years now and my wrists feel decades younger. My right wrist now receives 50% of the wear it once did. This number seems to be within my body's wrist-healing threshold; whatever damage I'm doing, I'm healing just as fast. I'll admit, it's no panacea (please continue to take breaks, exercise, and work on your posture), but gosh, it's such an easy, mindless improvement I'm surprised it's not done more often. A second mouse is way cheaper than carpal tunnel surgery!
It's totally possible to switch a single mouse from side to side, but it takes more effort, and I don't want to spend effort. Any context switch, however small, serves as a small impediment. By having two mice, there's one less thing to think about; I don't have to worry about my wrists, the second mouse takes care of that for me.