You'll get used to the left-hand position of the mouse in no time---that's one of the first changes I made when my shoulder problems kicked in.........

Also, consider swapping out your mouse for a touchpad. You don't grip a touchpad, the way you do a mouse. Instead, you just slide your finger lightly across the surface. So much of the pain of using a mouse comes from contracting your hands and fingers constantly......The touchpad immediately made a huge difference for me. In fact, I recently housesat for friends and within ten minutes of using their standard-issue mouse, my arm and shoulder were begging for a rest. Made me appreciate my touchpad all the more.........My touchpad is called the Easy Cat by Adesso. I think it cost me about $30-40 on eBay. Takes up no more room than a mouse, and doesn't have to be rolled or moved around..........Good luck!