This is going to sound a little bit weird, but I'd argue that unless you're not quite human, you'll always make tons of mistakes on GMAT questions. The key is making sure that you catch them before you click "next."
On quant, try switching up the rhythm of your practice to see if you can get rid of the careless errors. Before you start scribbling numbers, read the question twice. Pay special attention to any silly modifiers: if you miss the "x is an integer" or "x is two digits" or some other small nugget of information, you'll probably miss the question.
As you're finishing up your work on the question, take a few seconds to recheck any simple algebra and arithmetic that you did--just make sure that you didn't flub a negative or something silly like that. And then re-read the question twice more before you click "next", and ask yourself this question: "how are they trying to screw me?" If you've done enough practice, you've probably fallen into plenty of traps... and if you take a few seconds to think about it, you'll often be able to identify the trap--even if you just fell into it.
I know that this sounds like you're adding tons of time to each question, but none of this really takes very long. Yes, I'm asking you to read each question four times, but that rarely takes long on real GMAT quant questions (which are generally shorter than MGMAT questions, on average).
And you can't afford NOT to recheck your work. Because of the way the adaptive algorithm works, you get disproportionately punished for missing easy questions, and your #1 goal is to make sure that you don't miss easy stuff. You've probably seen this in your MGMAT tests already, but it doesn't take very many "easy" errors to send your score plummeting, especially if those errors appear early in the section.
On verbal, are you using process of elimination on every single question? If you fall in love with a "right" answer, you'll make plenty of mistakes; if you religiously identify four wrong answers, you'll make far fewer careless errors. I couldn't tell from your post if the careless errors were mostly on quant, or on both sections.
One last thing: when you review your practice tests and homework sets, continue to beat yourself up over the careless errors. You can miss all the hard questions you want on the GMAT (within reason), and still get a great score. You just can't miss the easy ones. So stay focused on your rhythm and accuracy, and you'll probably improve substantially on your CATs.
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