There are a number of ways to address this. One is to try using the mouse in your left hand. Another is to try a keyboard without a number pad, which means you can use the mouse in your right hand but with the right arm closer to the body. If you are an intensive user of the mouse this spreads the load and gives each arm a break. As well as the traditional mouse, you will find tracker balls, pen devices, graphics tablets and touch pads.
Different types suit different people, different hand sizes and different tasks.
Try out different ones and find the one that suits you. Use a suitable mouse mat Ensure that the mouse mat has a surface covering that allows free and easy movement. If you use a rest, it should gently support the wrist and you should use the whole arm to guide the mouse. Learn and use keyboard shortcuts If you find the use of the mouse a problem, virtually all programs and operating systems provide some keyboard shortcuts that will reduce the need for mousing. Adjust the mouse buttons via the software Use your mouse control panel to change the tracking speed, adjust the time interval for double clicking, use a click function to lock the drag control and swap over the buttons for right and left hands.