Cant Raise my score

Hey everyone
I really need some help with this I am a highschool student doing my Alevels i get fine grades in Alevels uually all As or atleast get highest marks in the class in some subjects but the thing is i cannot get a good SAT score many kids way behind me in class are getting much higher than my score and i am still in 1700 to 1800 range my weakness is Critical Reading whatever i do i cant increase my score i have done so many practice test and tried all strategies i could find and still whatever i do my score dosenot increase at all it stays in the same range of 500-600 and never ever increases i cant help it and its making me depressed and somehow when all others are better than me at it it is shattering me i really need help with this i really need a score near 2100 and i did give my SAT in october but due to intense pressure i got a head ache during the exam and screwd the whole exam and i am appearing once this DECEMBER… please anyone who can help me can save my life