I am not ADHD but I found with my Open Uni coursework, getting started was fairly easy, tying to get what I wanted to say condensed so it was detailed but also within the word count was much harder.
Another note from another course I did, was 'good is good enough' which is desiged to give us confidence that what we have written is good enough and we don't need to keep editing.
Keep up the good work, I am sure you will be fine.