Firstly try to find your weak areas and work on them.
In verbal, go for chunking instead of reading the whole long passage. You can also find some GMAT Ninja videos on YT on this topic.
For improving quant, you need to see why was the answer wrong, was it a calculation error, careless mistake, conceptual gap, or whatever reason. Write the reasons why you have done a question wrong (and how you can avoid making that same mistake again next time) on a piece of a piece of paper and try to avoid making that mistake again. Keep revising all wrong points every 3-4 days. Also, don’t forget to revise those points a day before exam.
Also, until you take exam try to make your study sessions longer (2.5 to 3 hrs), so that you will get used to keeping focus for a longer time.
Try to give mocks every 10 days in your first stage of prep. And every 3-4 days during the last weeks before your exam.
You can also try to switch the order to solve mocks. Sometimes the order to solve sections matters a lot. For me, i solve DI first (for me DI is tiring & tough) then 10 min break, then Verbal and Quant (which are easier for me and i can maintain focus while doing these sections)
To cope with negative thoughts, try to meditate and do some physical exercises everyday.
Hope it helps
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