It's also just harder to work from the computer - you have to keep looking up and down, etc. :)
When you're doing problems from books, prop the book up vertically so that you get more practice with the "up and down" thing.
Also, part of the issue is likely, as you say, mindset. You know it's a "test" situation and you want to get everything right and you get nervous and and...
So part of this is making sure that you've got the right mindset going in. Have you read this?
https://www.manhattangmat.com/blog/inde ... lly-tests/Read it every day for the next week and then once a week after that. That will help with part of the stress because you know the goal is to make good decisions about what to do AND what NOT to do.
You may also want to take some steps to help ratchet down the test anxiety, since it will obviously be even more nerve-wracking to go take the real test. Try this:
https://www.manhattangmat.com/blog/inde ... mat-score/There are some additional ideas in this article:
http://www.manhattangmat.com/blog/index ... anagement/Re: your ADD, how has that affected you on similar testing situations in the past? One of the big things that I've seen is the need for additional time not necessarily to work on the questions but to take a break and then come back in re-focused. One of my past students was granted extra time specifically in order to be allowed to take extra breaks because of ADD.
Still, it is a lot of money, it's a long process, and there's no guarantee, so I can understand deciding not to go down that path.
Try the stuff above and let me know how it goes.