Can you get someone to take a picture of your arms (and maybe your whole torso) as you type/mouse the way you usually do, in your current setup? (Or even a video, if you want to get fancy.)
Second, a question: how much mousing do you do on a daily basis, and for what kind of activities? Are we talking about professional CAD/Photoshop use? 5 hours of video games per day? Just navigating around a web browser? What other kind of activity do you do with your hands? E.g. are you a serious tennis player or guitarist? A rock climber? ...
People here can then probably provide some more contextually relevant advice.
Absent that, my one-sentence #1 most important piece of hand-tool ergonomic advice (goes for musical instruments, sports equipment, keyboards, whatever else): Make sure your wrists are in a relatively neutral (i.e. straight) position. The closer to neutral your wrists get, the stronger and more flexible your fingers will be, and the lower your chances of injury.
Once your wrists are straight, try to adjust your setup so that other joints are also in a relatively neutral position, with as low a continuous/static load as possible. If you need to do some activity which requires a static load on some muscles, make sure to take frequent breaks, change positions from time to time, etc. Finally, make sure you get enough sleep, enough exercise, eat healthy, and don’t have too much family/relationship/job/etc. stress in your life, if you can help it.