In Hamlet Act II, scene ii, Hamlet remembers that he loves acting, but he forgets, until the end of the act, that he is to be an agent of vengeance.

Remember that from Act I and Act II, scene i, Hamlet has assumed the role of "Crazy Hamlet" in order to hide his secret persona of "Revenge Hamlet."  In reality, Hamlet does not want to play either role; instead, he is acting on the orders of his Ghost-father.  In reality, Hamlet is happiest as "Player Hamlet," a man of the stage, a lover of drama.  Here, when the players come, Hamlet remembers his love for acting:

I remember, one said there were no sallets in the lines...One speech in it I chiefly loved: 'twas Aeneas' tale to Dido...

Later, Hamlet remembers the words to a prayer:

As by lot, God wot,
and then, you know,
It came to pass, as most like it was.—

Hamlet remembers the faces of the old players:

By'r lady, your ladyship is nearer to heaven than
when I saw you last by the altitude of a chopine. Pray
God, your voice, like a piece of uncurrent gold, be not
cracked within the ring. Masters, you are all welcome.

Then, at the end, Hamlet remembers that he is supposed to kill Claudius.  During the Player King's monologues, Hamlet has forgetten all about vengeance.  In the scene-ending soliloquy he says:

O, vengeance!
Why, what an ass am I!

It doesn't take a psychologist to point out that Hamlet is happiest among the players and unhappiest among the King, Queen, and his advisors (Polonius) and spies (Rosencrantz and Guildenstern).  He remembers lines, prayers, and faces from years ago, when he was a student and actor, but he cannot remember that he is supposed to put on an antic disposition as an avenger.