Before you start beating yourself up for making what seem like "silly" or "careless" mistakes, here's a couple of things to keep in mind:
1) Remember who your opponent is. The test writers are experts at generating questions which bring out stress, confusion and time pressure that lead to many things including calculation errors.
2) In my experience with more than 500 test takers, beating yourself up is the worst way to inspire a change in your error rate
The best cure for reducing error rates on quant questions is what I call building a more resilient process during each question.
Here are a few examples of process changes that I drill with clients that help to reduce error rates:
1) Noticing the pattern of the types of errors you tend to make. And anticipating that you might make them again by writing down more detailed 'messages' to yourself when you spot them.
For example, if you tend to forget units or which variable you're calculating -- then box it in big letters in your workbook right at the beginning of the question
2) Use a more resilient process. Don't try to be the smartest person in the room when you're taking the test. Write down a few more steps in calculations and avoid doing to much just in your head.
This makes it easier to double check your work and spot mistakes and correct them.
3) Remember the value of a question you know how to do -- give it the extra few seconds of precision calculation so you can afford to get a harder one (you don't know how to do) wrong.
Be unemotional about silly mistakes. Tackle them like every other hurdle on the GMAT. With better in-test processes.
If you want to discuss further or need help how to overcome this and other process challenges that are holding you score back, feel free to get in touch.