Indeed, it's the mouse's fault. I switched to a trackball years ago and it's a vast improvement. Best one I've found so far is the Kensington "expert mouse" which has a wrist rest attached at the front.
Another option which has worked wonders for me is the RollerMouse.
You just use your thumbs to move the bar around without ever taking your hands off the keyboard. It's a bit awkward at first, but you get used to it quite fast.
Everyone thinks I'm crazy, but I still prefer to use a trackball as well. Particularly the "Microsoft Trackball Explorer", which unfortunately is no longer in production (so I'm treating mine like a prince so it never breaks)
You just use your thumbs to move the bar around without ever taking your hands off the keyboard. It's a bit awkward at first, but you get used to it quite fast.
[see http://ergo.contour-design.com/]