I'm right handed but I taught myself to use a mouse with my left hand when I had a data entry job a few years ago. I had to type handwritten information from official forms into a database and check off various details on those forms with a pen as I entered them. I found it a right pain to keep swapping between my mouse and the pen all the time so I decided to see if I could use the mouse left handed - I could - and I got my work done a LOT faster too. From then on I kept the pen in my right hand and the mouse in my left - which really annoyed co-workers who used my computer while I was on my lunch breaks and I'd get back to find it swapped round to the right side again!

Now I ALWAYS use a mouse left handed. Keep practising and you'll never look back :).