The analyses of a text #2

The narrator recalls his adolescence.
He and his friend Steerforth stay at his old nurse's Peggotty's hometown.
They visit Peggotty's elder brother Mr. Peggotty's every night.
Tonight, the narrator founds Steerforth sit before the fire alone in Mr. Peggotty's house which is made of a boat.
Steerforh talks to the protagonist; the narrator finds him in a gloomy mood.

Steerofth fell for the fiance of Mr. Peggotty's nephew and adopted son.

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"Tut, it's nothing, Daisy[the narrator's alias]! nothing!" he replied. "I told you at the inn in London, I am heavy company for myself, sometimes. I have been a nightmare to myself, just now?must have had one, I think. At odd dull times, nursery tales come up into the memory, unrecognized for what they are. I believe I have been confounding myself with the bad boy who 'didn't care,' and became food for lions?a grander kind of going to the dogs, I suppose.
[David Copperfield by Charles Dickens]
1. I'd like to know why it is "memory," not "mind."
2. I'd like to know what "for" means here.
3. And I'd like to know what "going to the dogs" means.
Thank you in advance for your help.