Executive dysfunction comes from a lack of available dopamine. Common advice is wrong. You need to provide your own dopamine before you can start. Otherwise you're trying to run your car on empty.

"But what if I still don't do it" well you already weren't getting it done anyway. Now you have a little treat. Try again later.

You deserve kindness and care even when you aren't being productive.

I usually suggest this technique as a "Reverse Pomodoro." In the original Pomodoro, you work for 25 minutes and then take a break for 5 minutes (the times vary, but that's the essential ratio). People with executive dysfunction often find this insurmountable, and they get even more frustrated, and then the task seems even more difficult. So instead, flip those times.

FIRST, spend 25 minutes doing something energizing and engaging that you like to do. Not scrolling social media passively, not watching tv, not napping. Try something like colouring, doing yoga, running/walking around the block, talking about your favourite tv show with someone in real time, playing with the dog or cat, making and eating a lovely sandwich, hula hooping, something active. Having a little treat absolutely falls in this category!

(on the subject of little treats: refusing yourself food until you do work is for fucking Puritans and you can be kinder to yourself)

Then, after 25 minutes (or however long it takes to eat the sandwich or finish the yoga routine, it doesn't have to be exact), spend 5 minutes writing (or doing whatever you're struggling to start). Most people can coax themselves into doing something they find difficult for five minutes, if they have already filled up the joy/energy/engagement bucket. You can put a timer on for the 5 minutes if you want, or if you find that annoying, just work for as long as you like.

The other key is: don't push yourself to keep going when you're frustrated or tired—that will just reinforce the negative belief that you already have, which tells you that this task is painful to do, and needs to be avoided. If you've commonly had to force yourself to do this kind of task, that's likely part of why you think of it as painful and have trouble starting it now. Also, you should just, at a basic level, try not to put yourself in pain for the sake of productivity. So just do it till the good feelings run out. Then start hula hooping or colouring again for another 25 minutes. When the tank's refilled, try another 5 minutes of work, if you can. Adjust times to taste.