Agreed that we all need to practice we're writing very cleanly and carefully, step by step and line by line, on our scrap paper. I'm guilty of getting messy at times, too, and then errors creep in.
A couple of questions / thoughts.
(1) Do you feel constantly pressed to work faster? If so, that rushing is going to lead to you dropping things and making the kinds of careless mistakes you describe. What can you do to alleviate that "rush rush rush" feeling?
Part of that has to do with knowing that you are actually making conscious decisions about what to do and what NOT to do. The goal is not to try to get through everything fast. The goal is to work methodically on the things that
are worth your time and to be able to identify when something is not worth your time so that you can bail (and save time for other problems).
https://www.manhattanprep.com/gmat/blog ... -the-gmat/(2) Was this a problem even when you were in school? Or did you make far fewer of these types of mistakes then? It has likely been a long time since you've done much "paper" math. This is true for nearly everyone studying for the GMAT - at least, at first! Do some paper math, every day, multiple times a day for 5-10m at a time, until your brain gets back into that mode of working. Try doing drills out of Foundations of Math (the book or the online problem sets). Or google drill sheets for any math topic that you want to practice, eg "solve equations drill" or similar. Tons of math teachers have put things like this online. (For their 11 year old students, yes - but they'll still be helpful for you!
)
If you did have a big problem with this in school, too, then you will need to take more concerted efforts. I would recommend finding a local middle-school-level math teacher who works with / tutors students who have ADHD, dyslexia, etc. This type of teacher would be able to give you very good recommendations and help you to build habits to overcome these types of errors.