Mentally exhausted!!!.....help me motivate!!...plz....?

i am a 16 year old girl with average intelligence and i have taken up science stream....i am trying very hard to study every time with more of passion and hard work but i am not able to score in my tests...the more hard i try the more i fall...i feel its of no use to put in more hard work because i am not getting satisfactory results as a result i have become too de-moralised....people around me feel i cant do anything as compared to my elder sister....because i am trying my hand at the same thing what my sister tried and she rocked it so well....she's been a topper every time no matter whatever she tries....even i wanna do the same and make my parents proud as she does it everytime and i haven't done it anytime.....i want to make my parents proud as they expect the same from me....plz plz help n motivate me so that i can perform well and fulfill my and my families dream!!...