Learning to accommodate your ADHD and cultivate good writing habits that don't lead to complete creative and executive function burnout is a tough thing to learn but is possible.
Nor does a lot of "hard deadlines" as hard deadlines only really work for ADHD when there's panic involved. We need the adrenaline surge, even sometimes when we're medicated. But that's not healthy and leads to quicker and more prolonged burnout the more you do it. Trust me; I've obliterated my mental health several times this way, and each time it gets harder and harder to bounce back from.
So, what does work for us? Well, like @natalieironside pointed out, finding ways to accommodate your "fun" brain needs is far better than trying to make your brain perform to a neurotypical standard that will cause quicker burnout and force you to rely on hyperfixations to get things done. There are several ways to do this, but my recommendation would be to set yourself small goals with:
A Good Reward System.
I've talked numerous times about needing to make your own dopamine with ADHD. Our brains just don't create enough of it, even when medicated. I use a sticker reward system for myself. It's a fun way to make myself feel better and provide visual feedback that shows progress is being made. Progress my brain would otherwise never acknowledge and continue to make me feel bad for "never getting anything done."
For writing, my suggestion is to set yourself small daily goals. 500 words a day is my go-to; every 500 words, I get a star. If I feel capable of doing more that day? Great. If not, also great! I met my goal for the day.
If that goal is too much for you? Also fine! Start smaller, train your brain to get used to doing a little bit every day. Small progress is still better than no progress.
This reminds me, don't worry if what you thought the story was going to be evolves over time and looks nothing like your original plot outline.
Making plans tend to be viewed as the holy grail of success for ADHD. That if we just write the thing down, plotting it out in meticulous detail, that for once, maybe, we might actually complete something. But the truth is, stories don't always turn out the way you thought they would, and sometimes they need to change from your original plan to be better.
For many folks with ADHD, this can feel like a failure.
We're often told that our inability to stick to plans is a character flaw. So the pressure to "stick with the plan" is real and can even become debilitating. Failure to conform can cause feelings of inadequacy, depression, and general "why bother, I keep fucking up."
But writing is a creative process, and creative processes need to be flexible. So should you plot things out or just write as you go? Honestly, whatever works best for you. I advise doing extremely rough outlining, but again, keep yourself open to the idea that this is flexible. It is not set in stone. You are in control of this, and you can change it as you need.
If you're really struggling with structure or trying to figure out where something is tripping you up, getting other writer friends to beta read can help. Alternatively, you can hire an editor for a manuscript assessment so they can tell you where your strengths are and where things are tripping you up. Sometimes it's good to have another pair of eyes go over things so you can get a fresh perspective and adjust accordingly.
Speaking of editors: Don't Edit During Rough Drafts. It's far too easy to fall down a rabbit hole of hyperfixating on trying to make something "perfect" the first time around when in reality, it's much better to view the project as a whole before you start chopping it into pieces. What you spend weeks editing might not even make the final cut at the end. Save yourself the time now, and save that process for later.
If you really need to edit something, do so at the end of a milestone, like a chapter. But also, don't let yourself get bogged down in it. If you notice something that needs to be changed, leave yourself a note in the margins using the comment feature. It'll make it easier to find things when you get to the redraft stage and also go back and strengthen areas that need it. Can't stop fixating on it? Time to take a break before your ADHD goes down a spiral.
Also, taking a break from your work is not a failure. Sometimes you gotta rest the old brain-meat and come back when you're less burned out.
Check Your Environment.
Are you getting enough stimulation? Or are you getting too much negative stimulation?
I personally can't focus in a minimalist room; my brain just keeps latching onto how empty everything is and starts running in circles looking for stimuli. I like my spaces to be visually interesting. But I also can't focus in a room that's too messy.
As I type this in bed, I'm HUGELY aware of the pile of clothes on the floor that needs to be moved to the laundry. By itself, it wouldn't be a problem, but with the other stimuli in the room (the TV is on, I've got Discord running, I'm on Tumblr, and a small dog is cute at my feet), it's so distracting I keep tabbing out of this post and looking for "soothing" extra stimuli which won't actually help me focus on what I need to do. I'm able to block almost all the other things out, but my attention keeps coming back to the clothes. It is a type of negative stimuli, and it's sapping my energy away from what I want to do.
This is why before I sit down to work every day, I look at my environment to see if it could be benefited from a 15-minute pickup.
Finding the right stimulating environment is almost as important IMO as finding a distraction-free environment.
But what can you do to make your environment happy-brain-stimulating? That will really depend on your own individual needs. Like I said, I like things to be visually interesting. But I also like to wear noise-canceling headphones to block out the hum of the fridge two rooms away. I also listen to video game music. It's specifically designed to keep you engaged when grinding away at tedious tasks. I do have "hype" playlists, but I use those more for getting myself in the mood to write than trying to sustain workflow. Other people use stim toys. ADHD advocate, BlackGirlLostKeys has a good range of stim toys in her store.
Hype Yourself Up:
ADHD folks tend to be unnecessarily hard on ourselves. We've been conditioned by a lifetime of "if you just applied yourself" or "but you're so smart!" to set ourselves up for nigh on impossible standards we wouldn't expect of anyone else.
Are you thinking of your work negatively? Do you put yourself down, even humorously? Stop. I know it's a coping mechanism; I do it too. But you need to stop. Training yourself to think negatively about yourself and your work, even as a self-defense mechanism, eventually sticks, and it will become harder and harder to psyche yourself up to do the thing.
It's normal to feel burned out and tired with projects; that also happens to NT people. But unlike NT people, we can't just push through. It can cause us real distress to try and work on things our brains are too tired to do. For me, it even makes my chronic pain exponentially worse. But if you want to work on the thing and are just having an "ooh shiny" day (only ADHD people are funny when they make that joke, NTs don't try it), hyping yourself up can be really helpful. I don't mean to induce hyperfixation--although sometimes that is unavoidable. It's just how our brains work. But rather, make yourself feel good about your work.
A common iteration I say to myself is, "fuck yeah, time to fuck around and find out" as I sit down to write a new draft.
I try to make it into something I will look forward to doing. It's a reward for getting all my other shit done. Got my laundry done? Time to go play around with my vampire son and write an incentive to social reform and/or riot.
Did the dishes? Time to go create a whole new world! Hell yeah!
You can also just straight-up play with your characters. People think it's hilarious that I write fanfic of my own work, but it's also an excellent exercise for working through things like writer's block or burnout. It can be very liberating to take your character out of their world and drop them into a fandom setting.
When a client tells me they're struggling with a character, I tell them to take them out to coffee. Take your character, drop them in a coffee shop. What's their order, how do they interact with the world? How would they exist in this alternate reality while still being true to their canon self? Doing this can not only be silly fun, but it can also help you get a better grasp of your characters and the defining traits you want to emphasize.
Basically, treat it like a fandom you love, and write like you love it. That doesn't mean it won't be hard at times or that you might fall out of love with something and decide to move on to something else. But it's infinitely better than approaching things with the mindest of "ugh, I'm not getting enough done/nothing I do is good enough."
Anyway, this post got waaay longer than is probably helpful, but I hope anyone who needs it will glean something that might be useful to them. Happy writing!