I can sort of see all sides here. I was diagnosed with ADHD in my 30s. I'm in my 40s now. I bitterly wish that I had been diagnosed when I was much younger, because then my life probably wouldn't have been such an absolute shitshow. I wish that my mum had put even a fraction of the effort into me as she did my autistic brother. But I've spent a lifetime just being considered a monumental fuck up.
I left home when I was 17, although there were 2 occasions where I ended up back at my mum's.
My mum has noted that I have never listened to or taken any advice she gave me, even when it was clearly right and sensible. Although there were times when it dawn on me years later and my mum would say "I told you that 10 years ago!"
ADHD symptoms never got better as I got older, they got worse. But I didn't know it was ADHD then. My mum and stepdad got me out of so many pickles that my impulsive self got into. And looking back now, this was the best thing they could have done for me. They must have despaired of me so many times, and even in my late 20s, I was still causing them to pull out their hair. (I should add that I was never ever verbally abusive to my parents).
I am medicated, and it is the best thing that ever happened to me. I can now almost function like a regular person. Your DD won't speak to her GP about meds, but if she has shared care, the GP won't have anything to do with it and will just refer back to whichever company diagnosed her. Sometimes you need to try different types of meds. I take 70mg of Elvanse and have 2 top ups of Amfexa.
What i have noticed since taking the meds is that I am probably autistic, but the ADHD symptoms were covering it up. This makes things worse as they conflict. ADHD hates routine and autism needs routine for example. So it's a case of both needs battling, but neither win, so I'm in a chaotic limbo.
Both my sons are autistic, one has ADHD as well, the other has severe learning disabilities as well. We all have very different needs. I am their mother, and I am their carer. My son with the learning disabilities will never be independent, so I will always be his carer. He is physically aggressive towards me, my other son is often deeply unpleasant towards me. I understand that it is part of their disabilities, so give some leeway. When DS2 is saying nasty things to be, I refuse to engage with him until he regulates himself again.
My mum was claiming carers allowance for my brother until earlier this year (he is in his 30s).
You won't get her to do the things you think are good for her (although they probably are), because her mind is too rigid to accept it). She is also 18, so you don't have the sort of control (for want of a better word) that you would if she was legally a child. If you stop trying to facilitate all these things she won't engage with, you aren't washing your hands of her, you're being sensible. If you can be there for her and not turn your back when she needs you, even when she's made the same stupid mistake for the 100th time, that will be a lot more helpful.
I do think it would be helpful if you were to look into ADHD a bit more. But from the point of view of ADHD adults. ADDitude is a good resource. There are also several Facebook groups where you can ask an autistic/ADHD adult questions. It does seem as though there are ADHD characteristics that you can't quite grasp, and expect your DD to act like a NT. She knows the pill will improve her mood, she has a prescription that needs picking up, but she won't do it. That's the ADHD. It's frustrating (for us as well). It's ironic that ADHD people are expected to remember to regularly take medication to help with the ADHD when remembering to take medication is a problem we need help with. What may help is signing up to Pharmacy 2 U so it gets delivered to your house. Maybe a pill sorter would be helpful where she can see it in the morning when she has breakfast so it becomes part of a routine.
(Just to add that planners/colour coded anything is one of those things that the NTs think should help us. For some it does i guess, but there are those of us that don't remember to even look at the planners etc, or begin to see them as demands/dopamine drainers that need to be avoided at all costs).
Which then reminds me why you find it hard to get her to do anything - it's because there's no instant dopamine hit from it. And that is one of the things about ADHD, we don't have the dopamine receptors (which is what the meds help with). For example, I had an engineer coming to look at the washing machine and the kitchen desperately needed to be tidied first. There is no dopamine to be found there, so I ended up deep cleaning down the back of the radiator cover (don't ask me why the dopamine is there, I just find it really satisfying).