If you can't get on with Voice Recognition then you may like to try a track ball input device instead of a mouse. Basically it's an upturned mouse with the rolling mechanism on top so you don't have to move the device round the desk.
I've searched the Internet and have found that logitech produce quite a few track balls, some with ergonomic features, but most rely on the thumb doing the work. The other product I found is a "Big Track" which lies flatter on the desk and seems to allow any finger/thumb or part of the hand do to the rolling. This is the web address:
http://www.techready.co.uk/Assistive-Te ... 4wodHGVZAQ
(If this posting splits the web address into parts at the end of the line you might have to paste the individual sections into the http:// field of your Internet browser).
I haven't used any of the products I've searched although I did try out a trackball many years ago when researching assitive technology. So I'm not recommending anything specific for you. If possible your should try before you buy.
Also I found information about assistive technology on the Directgov.uk website which advises that some folk don't have to pay VAT, etc. If you want to check this out then the web address is:
http://www.direct.gov.uk/en/DisabledPeo ... Technology
(again you may have to copy and paste in the different sections of the address into your browser.)
Good luck.... think of this as an adventure and I'm sure you will have fun