Why does Pip, with his head “full of George Barnwell,” believe that he is a suspect in a crime? Why does he worry that he unwittingly provided the instrument for the attack?

Pip felt that his "convict" might be resposible, as the shackles were cut with a file that he provided. After sleeping on it..... Pip thinks a bit more clearly.

With my head full of George Barnwell, I was at first disposed to believe that I must have had some hand in the attack upon my sister, or at all events that as her near relation, popularly known to be under obligations to her, I was a more legitimate object of suspicion than any one else. But when, in the clearer light of next morning, I began to reconsider the matter and to hear it discussed around me on all sides, I took another view of the case, which was more reasonable.