He doesn't doubt or question himself unlike Hamlet. I believe Hamlet admires Fortinbras for those two qualities. Hamlet himself questions whether or not the ghost of his father was tangible. He has to put on a play in front of Claudius and see what his reaction to the play is. He wants proof that Claudius is the perpetrator. Perhaps Hamlet is afraid of committing a sin himself? Or is he just trying to get over the anguish and get his emotions out of the way before he can do anything about Claudius? I believe its both as he is afraid of committing suicide which is a sin and he loves his father dearly so I think he needs a bit of time to heal his wounds - ''His canon 'gainst self-slaughter. O God'' (Line 132, Act 1, Scene 2). Saying this, if Hamlet really does have so much love for his father shouldn't he act straight away and seek revenge just like what Fortinbras would do? Hamlet admires Fortinbras for what he is - ''Led by a delicate and tender prince, / Whose spirit with divine ambition puffed /Makes mouths at the invisible event.'' (Lines 48-50, Act 4 Scene 4). He admires him for not being afraid of death.
However, in Act 4 Scene 4, Hamlet reflects on a sick society when he learns of Fortinbras' attempt to capture worthless land from Poland. But seeing Fortinbras' army in front of him, he criticises his delay in revenging his father's death. Hamlet believes he thinks too much and that is probably right. This is one reason why he hasn't killed Claudius yet. In one of Hamlet's soliloquies on page 169, lines 32 - 48, he berates himself. He is angered that he keeps thinking about the consequences of killing his uncle, as that in itself is a sin. He is angry that he lets his emotions in the way of his duty and he does believe it is his duty to acquire revenge on behalf of his father. ''Sith I have cause, and will, and strength, and means'' (line 45, Act 4 Scene 4). This quotation from Hamlet states that he has a cause and a purpose now.