The note, which was written by one of John Robertson's rape victims and not sent was found by a family member in 2002, who read the heartbreaking letter and police were called in.
The letter stated: "For the last seven years John Robertson has sexually abused me. I am ashamed and embarrassed. Can you help."
At the High Court in Glasgow, Judge Michael O'Grady QC told Robertson: "You have been found guilty of an appalling course of conduct."
Robertson, 45, will be sentenced later this month.