Hi herenow,
I'm willing to bet that your child is rather bright and has been able to compensate/adapt until now. Like most of us, we know what 6-2 or 2+7 is, under normal conditions but under stress or tiredness or other things, it just gets slower and we easily make more mistakes.

Here are some ideas of what seems to be working for us:

- Write out everything, don't do it in your head. Especially when tired or stressed like on a test. Recognize your own body's feelings.

- Is she allowed a 'working' paper in math test? Or use the back of the 2nd page, for example... or just off the edge of the margin as a working area.

- When doing 6-2 does she prefer horizontal format or vertical? Write out things in the way she likes to see it.
- When writing out 6 - -2 go ahead and write it out and the mark it as a plus visually to show that two negatives make a plus

Maybe play some stressful timed games to get use to the feeling of being under pressure that is unrelated to math. Then do some math drill games for fun and just try to get better and better at it. Give self a reward when reach a reachable goal. The goal could be to get 2/10 right under 15 seconds and not break down from the stress and the reward will be ... . It is mastering the internal feeling. This is an ongoing technique that even adults find extremely useful in overcoming difficult problems.

best wishes