SFist has the PDF and full letter. Mehserle wrote it four days before the verdict.
Excerpts.
For now, and forever I will live, breathe, sleep, and not sleep with the memory of Mr. Grant screaming “You shot me” and putting my hands on the bullet wound thinking the pressure would help while I kept telling him “You’ll be okay!” I tried to tell myself that maybe this shot would not be so serious, but I recall how sick I felt when Mr. Grant stopped talking, closed his eyes and seemed to change his breathing.
Then why did you shoot him?
The Department of Justice has opened an investigation into the shooting, just like they did for the Rodney King beating trial