Tyler Lambert and Dana Plato
Jerry Laizure/Getty, Everett
Joni Richardson says her 25-year-old grandson Tyler Lambert, who committed suicide
May 6, was never able to come to terms with the 1999 death of his mother, Diff'rent Strokes
star Dana Plato.
"At 15, he started to get wild," Richardson tells PEOPLE, adding that Lambert had problems with drugs and alcohol, resulting in several rehab stints. "These past 10 years have been pure hell."
According to Richardson, many of Lambert's troubles stemmed from a desire to "drown out the pain" of losing his mother when he was 14.
In a note she found in his bedroom after his death, Richardson says her grandson wrote, "The only moments of happiness for me are when I'm real high and don't think about the past or I'm real drunk and not able to."
He even immortalized his pain with tattoos, she adds.
"He had his mother's name on his hand on his arm and right across his finger," she says. "He was putting her ashes in with the ink."
"He said, 'Joni, I have to do this,' " she continues. "[That way] he could feel that he had part of her with him."
For more on Dana Plato's troubled son, pick up a copy of this week's PEOPLE, on newsstands Friday