If no one could have gotten it done then your boss really shouldn’t object to the accommodations you put forward. If they do remind them how big of a job it is and that you have been doing all you can on it.
As for telling your boss about your ADHD, I struggled with this as well mainly because at the time I was diagnosed our boss left and I got a new boss. My old boss was family so she knew about my weekly doctor appointments. I did end up telling my boss only to show her that I wasn’t slacking off by going to regular appointments, but I understand if your afraid your boss might think your not up to the task. It really depends on the relationship you have.