And although Kate was aware that he had been fighting depression for many years, she admitted she was "shocked beyond belief" by what had happened.
She added: "A very distressing thing for me is that Steve and Shane were both 48 when they died."
Messages of sympathy for the widow of Constable Stephen Carroll, murdered by the Continuity IRA (CIRA) 11 years ago, have been flooding in from across the world.
The slain officer's death was a catalyst for change in Northern Ireland, with the then First Minister Peter Robinson, deputy First Minister Martin McGuinness and PSNI Chief Constable Hugh Orde standing together outside Stormont Castle to condemn the killers.
Mr Carroll was shot dead in Craigavon two days after sappers Mark Quinsey and Patrick Azimcar died in a Real IRA gun attack at Massereene Army Barracks.
And Kate, who became a high-profile advocate for peace, said she never thought she be going through a similar grief again.
"I saw Shane for the last time - he was in his coffin," she said.
"My heart is breaking - but at least he looks at peace now."
She added: "When I lost Steve I thought there was no pain like it...but now, losing Shane, I'm in the same position again."
Shane's funeral service will take place at St Patrick's Cemetery at 12.30pm on Saturday.
Shortly after his death earlier this week, heartbroken Kate told the Belfast Telegraph that Shane, her only child from a previous relationship, had failed to recover from the trauma caused by the brutal death of the man he always called Dad.
Kate (69) also told how he bonded instantly with Mr Carroll, her soul mate and husband of 24 years, but she also said Shane struggled with depression in the aftermath of his stepfather's horrific murder.
When Constable Carroll was shot dead by the CIRA after answering a bogus emergency call in March 2009, he became the first PSNI officer to die at the hands of paramilitaries. Kate said Shane "used to spend hours" visiting his grave, which is in a cemetery that can be seen from her Banbridge home.
He leaves behind 10 children - Jake (21), Dean (20), Katelyn (13), Jordan (11), Skye (7), Carly-Rose (6), Riley (5), Arianna (4), Harley (3) and two-year-old Ella-Rose.
South East Fermanagh Foundation director of services Kenny Donaldson said he was devastated to learn the "horrific news" of Shane's untimely death.
"Our hearts are breaking for Elaine, Shane's children and his dear mummy Kate," said Mr Donaldson.
"The Carrolls are much loved and respected members within SEFF."
Mr Donaldson said he spent time at the home of Shane and Elaine last Christmas.
"In the times ahead the SEFF family and others will do all that we can in supporting the family through the turmoil they will surely have to face," he said.
"We pray that God's comfort blanket will surround them all and hold them tightly together."
Mr Donaldson added that "those who murdered Stephen Carroll hold a level of responsibility for this tragedy".
"The emotional and mental turmoil Shane had to carry since the brutal murder of his stepdad and best friend has been immense," he said.
"The legacy of terrorism and violence destroys lives."