Hi
One critical tip that I share with all my clients is how to think of "silly mistakes".
Using the term "silly" immediately puts you in the wrong mindset of dealing with it. Recognize a couple of things:
1) the GMAT isn't a normal test -- It's more like sport. There's a vicious opponent on the other side (the test writers) who know your weaknesses and are looking to take advantage of them. Your job to compete well is to recognize their tactics and brace for them with stronger in-test processes.
2) each of us has our strengths and weaknesses -- that's just how it is. Calling some of them 'silly' is counter productive. It's better to be the smart one who focuses on and clearly identifies areas in which you tend to make repeated mistakes (eg. capturing information from prompt, calculating the wrong variable or errors in dividing decimals) AND then puts in place improved processes (steps) that help to counteract that tendency. The organized note taking is a great point made above. Another example is putting a big box around what variable you have to calculate FURTHER down the page you're taking notes on -- so you remember when you near the end of the calculation. There are lots of examples of steps like that.
Your target score is closer than it seems because you HAVEN'T done everything you can to improve. You may have done hundreds or thousands of practice problems but if you're not focused on improving your 'cross-question' processes (that apply to all quant or all verbal questions) then you're leaving 20, 30, 50 or even more points on the table.
I'm happy to discuss more specific tactics you can make in the short term if you want. Good luck with the online test.