Louise Speed found him dead at their home in November 2011 and at the inquest the coroner said he was not satisfied Speed intended to kill himself.
Since then, Louse has found letters where the former Wales manager opened up about his mental health struggles.
In it, a 17-year-old Speed wrote: "I'm so depressed. I'm just going to sleep now and hope I never wake up."
Louise Speed said she found the letter, written while he was at Leeds United, during research into a new book about the Speed family in an interview with 'The Mirror'.
"Seeing that was a light-bulb moment for me, in many ways. It answers an awful lot about why he did what he did. It's not something a normal 17-year-old would write, is it? Or not a well one," she said.
"Maybe the mental illness or depression was always there from an early age and it's been ruminating all his life.
"If so, us having Gary until he was 42 was maybe a bonus."
September 8, 2018
During a 22-year career, Speed played for Leeds United, Newcastle United, Everton, Bolton Wanderers and Wales, for whom he won 85 caps.
Speed later went on to manage Sheffield United and the Welsh national team.
July 24, 2018