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 letter.
The letter stated: "For the last seven years, John Robertson has sexually abused me. I am ashamed and embarrassed. Can you help me."
But it was nine years later before Robertson, 45, was charged with his crimes after other victims came forward.
At the High Court in Glasgow, Robertson, who shows guinea pigs, denied all the charges against him and claimed his victims, now women, were making up lies in order to claim compensation.
Judge Michael O'Grady QC told Robertson: "You have been found guilty of an appalling course of conduct, years of preying on vulnerable young women time and again.
"Your behaviour blighted their childhood and haunted their adult years."
The offences were committed between 1990 and 2002 at Mansefield Farm, Garshangan Road, Kilmacolm.
The victim who wrote the letter is now 26 and she told advocate depute Jane Farquharson, prosecuting: "I can't remember how many times I was raped."
The woman added: "I thought that it kind of happened to everybody and once I realised it didn't I thought I should have said something sooner, but I didn't know how to stop it."
Another victim said she 'just froze" while he touched her inappropriately.
A further charge of rape was found not proven against Robertson.
Robertson was remanded in custody. He will be sentenced later this month.