Are you holding your left hand stiffly because using it that way (especially mousing) involves unfamiliar motions or awkward positions?
Does it help to pay attention to technique, as you might pay attention to your breathing and specific use of muscles when lifting weights?

Can you listen to relaxing music while you work? More than half the people around here have headphones on when they're at their desks. I don't...the listening-to-music part of my brain overlaps too much with the writing part and the calculating part. You may want to give it a try, at least for a little while, and see if it helps any.

Using keyboard and mouse *continuously* will make any kind of RSI worse. Try to take a short break every 15 minutes or so, and stretch your fingers and hands. *hug* Good luck.

Around here, craft stores sell these horrid looking orange spandex things that look like fingerless gloves. They are marketed for cross stitchers. I find using them can help my keyboarding and mousing aches and pains quite a bit.