Need some advice with CR

Hello everyone!! My SAT is on 3rd October. It will be my first & my final time. My Critical Reading score is stuck between 540-600. Please note that I get 17 out of 19 sentence completion questions right. My main weakness is time management. It seems that if there are two long passages in a section, I am unable to make time for both. In average I fail to answer 6-8 question in each section of CR. Any useful advice for me for better time management? Thanks in advance!!!