Find a doctor that understands the difference between using cannabis therapeutically, and actual life-ruining, physically addictive drugs.
Be up front right away. Tell them that it’s essentially medicine for you, and without it, whatever you are seeing them for (presumably ADHD) gets exacerbated.
Don’t get defensive if they bring up the psychological addiction to cannabis. Freely admit that you may rely on it psychologically at times, and maybe ask them if they drink coffee every morning (or if they have a problem with you drinking coffee every morning).
Literally every single person in rehab for actual serious drug addiction, lives on caffeine and nicotine. It’s all a massive double standard. The only difference is that, while medicating you, cannabis can also make you feel good (but usually not good and productive), and there’s this puritanical concept that this somehow makes it inherently bad and dangerous
Anyway, find a doctor that understands this. They exist.
Then go on and test positive for cannabis monthly, it’s quite freeing. As long as you don’t give them any reason to be concerned about your cannabis intake, then they likely won’t bother you about it much again. Maybe mention it every few months.