When does Scrooge realize that he was supposed to learn a lesson from the past in A Christmas Carol?

Scrooge is told by Marley that the ghosts will lead to his reclamation, and he is reflective several times during the visits to his past.

Jacob Marley tells Scrooge that he has arranged a visit by three spirits as a “chance” to escape his own fate, which was to be miserable and hopeless after death as a ghost.  Even though he resists, Scrooge is struck with emotion from the first image he sees of his past self.  Soon he starts to reflect on how he has been behaving in the present.

Although Scrooge does get emotional when seeing his childhood self, the ghost has to show him a few images of the solitary young Scrooge all alone at Christmas time before it has any real effect.  Scrooge’s first acceptance that being shown the past has meaning is when he reflects on how he treated the boy singing carols.

“What is the matter?” asked the Spirit.

“Nothing,” said Scrooge. “Nothing. There was a boy singing a Christmas Carol at my door last night. I should like to have given him something: that's all.” (Ch. 2)

Seeing himself as a child, and remembering the hard life he had, has helped Scrooge generate sympathy for children.  He compares himself to the boy signing carols, whom he harshly treated.  He begins to wish that he had treated the boy differently.  At this comment, the ghost “smiled thoughtfully.”

When Scrooge sees his younger sister again, he again reflects on the present in comparison to the past.  The ghost reminds him that his sister had a child, and Scrooge grows “uneasy in his mind” when thinking about how he treated his nephew when Fred asked him to dinner.

At Fezzywig’s warehouse, Scrooge sees his younger self enjoying a party and is lectured by the ghost on how much enjoyment was created for “but a few pounds.”  This again causes Scrooge to reflect on his treatment of those around him.

What is the matter?” asked the Ghost.

“Nothing particular,” said Scrooge.

“Something, I think?” the Ghost insisted.

“No,” said Scrooge, “No. I should like to be able to say a word or two to my clerk just now! That's all.” (Ch. 2)

After seeing how his former boss treated him, Scrooge suddenly realizes that he does not treat Bob Cratchit very well.  He feels badly about this, again, and considers what he might say to his employee. 

The only time Scrooge seemed happy was short-lived.  Following the images of the party, Scrooge is shown something he definitely does not want to relive: the time he broke up with his fiancé.  After this memory, Scrooge is forced to see Belle as an older woman, married and happy.  He does not want to face it again.

“Spirit!” said Scrooge, “show me no more! Conduct me home. Why do you delight to torture me?”

“One shadow more!” exclaimed the Ghost.

“No more!” cried Scrooge. “No more. I don't wish to see it. Show me no more!”  (Ch. 2)

He does not get his way.  The Ghost of Christmas Past makes him go.  Although he cannot help but be curious about what Belle looked like older, to hear them discuss him sitting alone while his partner is on his deathbed is too much for Scrooge.  He forces the ghost off, having had enough.

Forced to take part in Marley's experiment to reclaim him, Scrooge has several moments of self-reflection and repentance during his visits to the past.  He thinks about the poor boy, his nephew, and his fiancé.  Although Scrooge is by no means reformed, he has been forced to think about his past and learn from it.  He has two more ghosts to go through, and with each one he gets more and more receptive to the lessons they have to teach him.