Getting back to the OP, I've experienced something similar to this on a regular basis. Most people are cool about it but there are always a small few who when I mention I have ADHD imply I'm somehow 'backward' - that word even being used once. Another scenario is they'll suggest (or even state) it's just an excuse for poor/disruptive/irresponsible behaviour. I'm cool with it by now (although that doesn't excuse it) and when they tell me I'm imagining it and that it's "all in your head" I reply, "It's a neurological condition, therefore of course it's in my head. Where would it be, in my ankle?"
Having never received a suitable comeback from that, I always enjoy the look on their face as they slink away
