If your problem really is just careless mistakes, then it's very simple. I had a similar background at college (math & engineering major) and all I cared about was my silly errors. Here's how I did it:
1. Find a set of tricky math problems. I had a curated set of ~160 questions that represented all the traps one can see on GRE.
2. Do them. And when you do,
be as careless as you can. At this point, you want to EXPOSE all the careless mistakes you can possibly make. A perfect GRE math score does not allow ANY careless mistakes.
3. Take a look back and figure out why you made those mistakes, figure out how to avoid them in the future, make a list of actions you can take and practice them with some new questions. You want to form good habits at this point.
4. Take a look at the list you created at Step 3 the morning before the test. You got this.
This is literally what I did for my GRE math. My whole prep process for that was just one day. I made every single possible mistake I could have (I got 30% of my practice questions wrong, partially intentionally), and never made them ever again. The real test felt like a breeze afterwards.
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Yuan Liu, Creator of Gregory. Dartmouth Grad.For More GRE Math Resources