ADHD Brains are different - so some advice that works for non-ADHD brains may or may not work.
In general, being present and meditation (in whatever way that works for you, but, generally the practice of observing your thoughts as they go by but not reacting to them) are helpful for ‘strengthening’ your thought patterns. Becoming aware of things and building up that muscle is how you can have more of that willpower.
Habits can be very difficult to build, don’t get discouraged. Find things that work for you and ways to incentivize or motivate yourself.