What is the source of Hamlet's self doubt?

In William Shakespeare's tragedy Hamlet, the titular protagonist Prince Hamlet is wracked by doubt in himself, his perceptions, and his decisions. 

This self-doubt has one overarching cause, that being the decision he should make in regards to avenging his father. King Hamlet's ghost, or what claims to be his ghost, appears to Hamlet and tasks him with revenge for his murder. Hamlet struggles with this task: he is uncertain about his uncle's guilt, the...

In William Shakespeare's tragedy Hamlet, the titular protagonist Prince Hamlet is wracked by doubt in himself, his perceptions, and his decisions. 

This self-doubt has one overarching cause, that being the decision he should make in regards to avenging his father. King Hamlet's ghost, or what claims to be his ghost, appears to Hamlet and tasks him with revenge for his murder. Hamlet struggles with this task: he is uncertain about his uncle's guilt, the accused murderer; he feels conflicted between the morality of murder and the justification of revenge; he is unsure if what he is perceiving is reality or not. 

As a result, Hamlet spends much of the play trying to ensure that the decision he is making is the correct one. He in fact chastises himself for this later in the play when comparing himself to Fortinbras, a foil character who acts before thinking.