Piano technique troubles with tremolo - help?

I started practising The Pathetique by Beethoven quite a while ago - however, I cannot get past the Allegro part, as my left hand gets stiff and starts hurting during the tremolo. I never used to have troubles with it, but now I am terrified of it :) I've tried various technique excercises to make it better, but nothing seems to work and it makes me sad and frustrated... I really want to play The Pathetique! Do you have any suggestions?