Please explain this quote from Great Expectations by Charles Dickens: But that, in shutting out the light of day, she had shut out infinitely...

Pip narrates that in becoming a recluse from the world, Miss Havisham has prevented herself from the availability of the healing qualities of nature and friendships. Therefore, she has become so eccentric that she is no longer mentally stable.

This quotation is pulled from Chapter XLIX of Great Expectations in which Pip responds to a note written by Miss Havisham that is given to him by Mr. Jaggers. Since she has asked Pip to visit her in response to Pip's request to talk to her, he returns to Satis House and finds Miss Havisham in a different room from her usual one. After greeting Pip, Miss Havisham asks Pip if there is something she can do for him; Pip tells her of his plan to get Herbert into a partnership, and she agrees to financially support this effort.

Then, she asks if there is nothing that she can do for Pip himself, and Pip declines. So, Miss Havisham writes a note to Mr. Jaggers to release the money for Herbert to Pip. She hands him this note, telling him on the first sheet of the writing pad she has a place for him to sign his name, under the line "I forgive her" that will attest to his forgiveness for the cruelty she dealt him in his youth.

“O Miss Havisham,” said I, “I can do it now. There have been sore mistakes; and my life has been a blind and thankless one; and I want forgiveness and direction far too much, to be bitter with you.”

Nevertheless, Miss Havisham feels tremendous guilt for teaching Estella to be cold and break Pip's heart. She exclaims, "What have I done? What have I done?" Pip does not know how to respond to her. Of course, he knows that her teaching Estella to be cold-hearted is wrong. But, what she has done to herself by shutting herself away from the world in darkness is terrible, too. For, she has caused her mind to become diseased--

...that, in seclusion she had secluded herself from a thousand natural and healing influences; that, her mind, brooding solitary, had grown diseased, as all minds do and must and will that reverse the appointed order of their Maker.

In seeking revenge against men by isolating herself and instructing Estella in cruelty, she has greatly harmed herself. Indeed, her eccentricities have deprived her of love and friendship which could have healed her. But, now it is too late. These "vanities," Pip observes have destroyed Miss Havisham.