Hello! Well, I need help with Critical Reading! I need to raise this score DRASTICALLY! I am pretty good at sentence completions, but I cannot do the passages! I underline key lines/passages, yet I still fail terribly!
Soph PSAT: 45, Junior PSAT: 46, Junior SAT (March): 46.
What is some advice to ace this section?
My advice would be to go through the practice tests in the blue book, or to take some practice tests online. Also, there are many APP’s out there that focus solely on SAT Vocabulary words - which is crucial for the Reading Portion of the SAT. Happy studying, and good luck! 
If you are willing to put in the work, there is a very thorough practice book by Erica Meltzer that you can get on Amazon called the Critical Reader.