Self-determination theory (SDT) has been the most useful theory for me. Here's a link to a free online course starting 12/21
https://www.coursera.org/learn/self-determination-theory?
There's also a very good book by Ziviani that takes an SDT perspective. It's about kids but these principles are, essentially, universally applicable to all ages and disorders
https://www.amazon.com/Art-Science-Motivation-Therapists-Children/dp/1849051259/ref=sr_1_1?dchild=1&keywords=ziviani+motivation&qid=1607810946&sr=8-1
Ziviani also has nice book on goal-setting.