“How much of my ungracious condition of mind may have been my own fault, how much Miss Havisham’s, how much my sister’s, is now of no moment to me or to any one. ”
It all comes down to the italicized portion. No one is to blame, everyone is to blame, but it doesn’t really matter because… “The change was made in me; the thing was done. Well or ill done, excusably or in- excusably, it was done.”
He accepts that he has a part in it, just as everyone has a part in what’s going on around him just as everyone around him has contributed as much as he has to the current state of affairs in their way. He can’t blame it on others or entirely on himself, but why bother to blame anyone when it cannot possibly change anything? He’s taking responsibility for his actions and moving forward because where he can’t do anything about the past, he can do something about the future.