I have thumb - or rapid-fire shifters - on 2 of my bikes and I had to move the shifters. I had problems with my thumbs too and I realized it was coz I was pushing very hard and way out to shift, I was actualy pivoting my hand on the grips to shift. I just moved the shifters in closer to my hand and pivoted them so they sit a little higher. Now my thumbs don't have to reach so far to shift and that has resolved my problem.
Life should NOT be a journey to the grave with the intention of arriving safely in an attractive and well preserved body, but rather to skid in sideways, champagne in one hand, strawberries in the other, body thoroughly used up, totally worn out and screaming: "Yeah Baby! What a Ride!"