Hope things go well for you tomorrow. I know my old boss had ADD. Honestly, his manager was not at all understanding of his disability; in spite of that, getting a diagnosis (which he got after his son was diagnosed & he learned it often runs in families) meant that he was able to access many resources that ultimately helped him transfer to another position in the company & find much success in that position (which paid roughly the same, but was a slower, more steady work flow and less detail-oriented).
If you're interested, some of the resources that were helpful to him were:askjan.org/media/lear.htm
(awesome resource, you can also phone them)www.livingwithadd.com/tbiresources.com/tools/organizeday.shtml
(for TBI, but applies for ADD also)
He also found an ADD coach. The psych who tested you should be able to help you find one in your area, but here's an example of what they do: www.addcoaching.com/
Finally, he set up processes -- with the help of his coach -- to create reminder, checks & redundancies that together improved the quality of his work. When he first was working on that, he set up lots of reminders on his PDA & Outlook. He used those to help him stay on task.
After implementing all of those resources, the only issue he needed help with was communicating with clients. As his secretary, I prepared a list of talking points so that he & his clients could work together to ensure he stayed on topic & talked about
the points in order. He did find that he really could not do a job that required over 99.99999% accuracy. It was just too much. But now he has a similar job where 95+% accuracy is acceptable and he has found much success.
I would say that if you are diagnosed with ADD, then ask your therapist to help you sort out whether or not to tell your boss. It is not a choice that has to be made tomorrow, is it? Has your boss given you a formal warning that you must get it together by a certain date? If not, I'd hold off on telling your boss until you find a coach & start to put together your list of resources. The ADA does not excuse disabilities. If you cannot do your work even WITH reasonable accommodations, then your company can terminate your employment. On the other hand, if you go in with a diagnosis & a definitive game plan, you are much more likely to get the help you need.
But I know from my former boss, coming up with that list was not easy for him. He would come up with a few ideas, lose his list, try again to come up with a few ideas, get distracted & start on some other project, start talking with one of his children, and forget all about
the list until the next day when he was struggling at work. Then, he would work on his list, rather than doing his work & got in trouble for that. By contacting JAN and finding an ADD coach, he was able to start to set some goals & wrote them down in his Blackberry. He set aside a block of time each day to work on his list and after a couple months, he raised the issue with his boss. It was stressful talking to his boss, but by having his list & a support system in place, it went much smoother.
Hope that helps! Good luck tomorrow.