well, i find that mouse very comfy. but the sad answer is .. it's the wrong mouse for your hand. trade it to someone and buy a different mouse. it's not uncommon for this to happen, lots of ppl (me included) just can't use some mice.
it would be great if you could try different mice before buying.
Try putting a decent-sized wad of blu-tac on top of the mouse on the side nearer your last two fingers,
sometimes the extra stretch removes the pain from cramping your hand up, if it does help you then you can replace the blu-tac with some epoxy putty thats more permanent and durable