What does Cassius want Brutus to realize about himself?

I think you are referring here to Cassius's speech to Brutus in act 1, scene 2 of Julius Caesar, in which he prevails upon Brutus to recognize that "the fault, dear Brutus, is not in our stars / But in ourselves, that we are underlings." That is, it is not because of fate that Cassius and Brutus have lesser positions than Caesar has, but only because they have not agitated against Caesar. Cassius describes to Brutus a story of how he once rescued Caesar when swimming as an example of Caesar's humanity. He suggests that for Caesar to "bestride the narrow world / Like a Colossus" is unjust and that it is only because Brutus cannot see his own reflection that he does not realize he is just as worthy to command as Caesar is.
As Cassius puts it, "why should that name be sounded more than yours? . . . Yours is as fair a name." Brutus, Cassius tells him, is just as worthy, as "heavy," as Caesar and is capable of leading a revolt against Caesar's dictatorship.
Cassius offers himself as a "reflection" to Brutus as an attempt to show him his own "hidden worthiness."