Pip’s visit to Miss Havisham’s house with Estella is painful because he finally realizes that Estella is heartless, and they will not live happily ever after.
By this point, Pip is finally starting to come to terms with the reality of his situation. The ugly truth is that Miss Havisham is not his benefactor, and he is not going to marry Estella.
I saw in this, that Estella was set to wreak Miss Havisham's revenge on men, and that she was not to be given to me until she had gratified it for a term. I saw in this, a reason for her being beforehand assigned to me. (ch 38, p. 205)
Miss Havisham accuses Estella of being cold and heartless. Estella scoffs, and reminds Miss Havisham that she should not be surprised, she has made Estella who she is.
“You should know,” said Estella. “I am what you have made me. Take all the praise, take all the blame; take all the success, take all the failure; in short, take me.” (ch 38, p. 206)
Estella finally throws what Miss Havisham has done to her back in her face. Miss Havisham shows some rare emotion here, seeming to want Estella to care about her. Estella is bitter from what she has become, and what Miss Havisham has made her.
Pip and Estella recreate their childhood games, but with new insight. They are both older and poised, and each fully understands the game that has been their entire lives, and resents it.
Pip has spent much of his young adult life making mistakes. He has grown into his expectations by becoming the worst parody of a gentleman. He has no concept of what a real life is. Now, as this final veil is removed, Pip has to come to terms with the reality of who he is and who Estella is, and who they are not.