This method requires a significant amount of executive function.
My body doesn't feel the passage of time consistently. So my mind is never prepared to switch activities when it needs to.
And there are times my brain stops working on a particular task and nothing can get it started again. It's like a leg going out, you just can't stand on it.
This isn't occasionally where habit could be picked back up. This has been a problem every day of my life.
In my experience, this has been the death of every bit advice I've gotten from a neurotypical person. A lot of them keep circling back to discipline or trying harder as a solution to a problem they can't make sense of. Lack of understanding isn't their fault, this is so far outside their frame of reference they can't make sense of it in a single conversation. Fortunately understanding isn't required, only the acceptance that other people have limits they don't have themselves.
I struggle daily to urge myself to eat after years of habitual starvation. The process of storing and making food through-out the week is extremely difficult for me to say the least... I also don't have room in my finances to out-source this completely. To combat this, I have been successfully meal prepping on the weekends; however, I still often struggle with the basic task of eating the food, prepped and served. It is a common experience for me to get part-way into eating a dish, move on to another task, and neglect the food until it's spoiled only to realize so when I pack up for the day. Sometimes I will even notice the food, deep into a task, but the thought to address it is hardly formed.
In this regard neurotypical advice _did_ actually help me I suppose. However, when applied to a habit not immediately linked to your existence, it is quite alienating to receive.
I would imagine you'd get this advice from other non-neurotypical people too. My son is neurodivergant, and strict routine like what is being described is about the only thing that keeps him able to handle most daily life tasks regularly. But plenty of other advice he constantly gets frustrates him similarly to the frustration you seem to be describing. We call it "you've just never had it cooked right" advice. So I feel your exasperation.
Just a shout out here for medication. ADHD meds are rated effective in the 70-90% range, which is just incredibly good compared to medication effectiveness for just about anything else.
I have ADHD, and hate the feeling of being a victim. "I have this, so I can't do that. It's just the way it is." No! Not for this. Not when there are so many treatment options.
I accept that things may be harder for me than a typical person, that I may have to put in more work than other people to get the same results, that this is something that's very real that I have to deal with and manage at all times. That there will be times when I will fail and my stupid monkey brain will win the moment. But I won't let it define me, I won't let it dictate who I am and what I can and cannot do.
EDIT: Also, I mean to agree with you here: there's a point where no amount of discipline will work, and the advice to "just try harder" sounds like an alien telling you to just grow wings and fly. If you find yourself at that point, medicine can and will help. It also helps you be able to get in a routine of actually doing exercise, which in turn helps even more, and it becomes a sweet positive feedback loop.
Do you ever feel unmotivated to go to the toilet despite needing to? Has this lack of motivation ever stopped you from getting from your chair?
What modern people usually lack is not time, but lack of energy. Usually this is thought as the energy to do stuff (like coding a side hussle in the evening). But often it manifests in a lack of energy:
1. to make a decision (to do something)
2. to slow down, to stop the current activity and to think with the rational mind.
So you need to recognize these things and do certain decisions beforehand to solve the problem. Stuff like:
1. Go to the gym in the morning, when you still have the decision energy.
2. Create a habit, linking a new habit with the old ones, in order to decrease the energy expenditure
3. Increase the stakes, like getting a gym buddy
4. Decide stuff beforehand. Pack the bag, set up the alarm clock (to go to gym, to go to sleep)
5. When you are tired, actually rest. Don't turn the tv on, don't scroll social media, stop touching yourself via phone. If you are tired, eat, go to gym or a walk, go to sleep or simply sit in your chair or lay on the sofa looking at the walls. I guarantee, watching at the wall for 30 minutes straight will give you great motivation to do something else more productive. Don't let the monkey in you convince you to do the unproductive things I mentioned. Stay strong and make a rational decision what to do instead of looking at the wall. Do the right thing, not the thing that may feel nice in the midst of it.
6. Take care of the nutrition/sleep in order to increase the energy reserves
I hope that helps.