You went back to what you knew
So far removed
From all that we went through

from Amy Winehouse – Back to Black Lyrics on Rap Genius

Meaning

As in the first line, he went back to his life before Amy, either in his lifestyle or his ‘new old'girlfriend, and he seems to have acclimated very easily too it, having forgotten everything he and Amy had been through, which after a fresh breakup, is a very devastating prospect; when you’re not really over the guy, and he’s over you, or makes it seem like you never even existed.

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