In The Lords of Discipline, Pat Conroy proves that only with the acceptance of individuality,can personal beliefs, desires and priorities define one’s true identity.

Is this a strong thesis?
How can I improve it?

I think that that is a very good thesis statement.

Now, to prove it , take a sheet of paper and divide it into thirds. On each third write one of your 3 major areas of proof. Then go through the book and find specific instances when that area was dealt with; write down the gist of that event and the page number. When you have done that and feel you have found all the support you need. Go back and develop each of those areas into a good argument. Then put your paper together.