What new philosophy motivates Hamlet in Act 5?

Hamlet has been so concerned through the first four acts
of play about making good choices and the potential consequences of all of his actions
that he has been slow to take action against Claudius and avenge his father's murder. 
But in Act 5 he realizes that he doesn't have control over everything in his life and he
needs to just respond to the moment.  He specifically says to Horatio that he realizes
that "there is a divinity that shapes our ends rough hew them how we will."  What he
means is that we humans can make choices and take actions that give shape to our lives,
but that there is a fate/destiny/Providence that is also in play that will also give
shape to our lives -- probably some of the finer points of our lives are out of our
immediate control.  Hamlet follows this thought up later when Horatio is cautioning