One of the best sources for figuring out which diet and what type of exercise plan your horse needs, depending on what type of PSSM they have, is a facebook group called Managing PSSM, RER and other muscle diseases. The person who runs the group gives a lot of time and effort to each person that asks for her help - you can join the group by clicking the link below:
TAKE ME TO THE FACEBOOK GROUP Managing PSSM, RER and Other Muscle Diseases
If you would just like to share experiences and knowledge with others going through the same things you are with genetic muscle diseases in horses the link below will take you to one of the best sources for PSSM knowledge in the world at this time: