I am, in fact, diagnosed with ADHD, and my previous therapist says I'm a fairly extreme case. I do have alternative strategies for organization, but have not heard anything about problems with cultivating habits (indeed, many people have recommend cultivating habits as a coping mechanism).
A lot of advice I hear is that it takes about 30 days of doing something daily to form a habit. Perhaps you are stopping just short of the necessary number of days?
Also, given that you have been diagnosed with ADHD, maybe it would take you longer than others to form the habit. I don't have any outside info to support that -- it's just a possibility that came to mind.
It may be difficult to cultivate habits with ADHD because of the need to do it consistently, and the struggle to perform something consistently while getting consistent necessary associative dopamine if I understand it.
