Lifting weights... imagine the stronger person you will become. Studying for that exam.. picture the career you aspire to. Avoiding that donut... imagine the healthier you.
Habit stacking helps to remind one of the task to do. To avoid the struggle of doing them, picture the desired outcome.
>Imagine and focus on the reward of these tasks.
IME if you have ADHD those two things are basically the same. Sure, don't think of the pain, but you could ended up stuck in daydreaming about the perfect future where you're healthier, stronger, happier, etc. You can trigger executive dysfunction (mental freeze) either way.
This seems extremely relatable.
Except instead of "I'm healthier, stronger, happier etc." it's more like "I've accomplished all the things I've thought of accomplishing" (each of which seems well within my grasp, if I could choose).
When I read 'The Secret Life of Walter Mitty' well into my adulthood, I cried.
If you focus on the end-goal you can easily get frustrated once you don't see any results, specially on such long-term goals such as graduating, improving your career or becoming buff by doing exercise.
Supposedly it's actually better to just try and find meaning and joy in the process itself, enjoy lifting/studying regardless of the end results.
Obviously none of this is an exact science so each person might think different.