I believe there is a Gemara or Mishna that says that each Jew should believe that the world was created for them alone. Does anyone know where this source is.
Bishvili nivra haolam Source: Tractate Sanhedrin Mishnah ch. 4:5
"....In civil cases one may repay the money damage and he is atoned; but in criminal cases the blood of the person executed, and of his descendants to the end of all generations, clings to the originator of his execution. So do we find in the case of Cain, who slew his brother. It reads [Gen. iv. 10]: "The voice of the 'bloods' of thy brother are crying unto me from the ground." It does not read "blood," but "bloods," which means his blood and the blood of his descendants. [According to others it reads "bloods" in the plural, because his blood was scattered all over the trees and stones.] Therefore the man was created singly, to teach that he who destroys one soul of a human being, the Scripture considers him as if he should destroy a whole world, and him who saves one soul of Israel, the Scripture considers him as if he should save a whole world. And also because of peace among creatures, so that one should not say: My grandfather was greater than yours; and also that the heretic shall not say: There are many creators in heaven; and also to proclaim the glory of the Holy One, blessed be He. For a human being stamps many coins with one stamp, and all of them are alike; but the King of the kings of kings, the Holy One, blessed be He, has stamped every man with the stamp of Adam the First, and nevertheless not one of them is like the other. Therefore every man must say: The world was created for my sake, hence I must be upright, just, etc. ...."
Our Rebbeim usually quote this to encourage self-esteem and personal responsibility.