A government minister says he has "vivid memories" of trying to save the life of a police officer killed in the Westminster terror attack in March.
Tobias Ellwood gave emergency first aid to PC Keith Palmer who was stabbed outside the Palace of Westminster.
In a Daily Telegraph interview, he said the hardest part of his experience was trying to explain to his eight-year-old son what had happened.
Five people died in the attack by Khalid Masood on 22 March.
Mr Ellwood attempted mouth-to-mouth resuscitation of the injured PC Palmer after he was stabbed by Masood, who had driven his car into pedestrians on Westminster Bridge.
The Conservative MP for Bournemouth East said: "I think the hardest thing, as well as stepping through with others to try and save PC Keith Palmer's life, was coming home and finding my eight-year-old boy on top of the stairs having refused to go to bed.
"It was 10 o'clock at night and he was really confused. He couldn't understand why a bad person would do what he did and he also couldn't quite understand why I had then stepped forward in the way that I did.
"I had to explain to him that there are some bad people in this world.
"There are bad people doing bad things, but there are more good people doing good things, and that's why we stand up to events such as this."
Mr Ellwood is now a defence minister and spoke to the Telegraph as the government launched a strategy which aims to improve the mental health of the Armed Forces, veterans and their families.
He said members of the Armed Forces worked in a "very macho environment" but urged war veterans dealing with traumatic experiences not to "bottle it up".
He told the paper: "What I went through is something... but we shouldn't forget that there are many people who have seen much worse and continue to be affected by it.
"That's why it is so important for us to have the mental health strategy that we need - a veterans' support package that is understood and a covenant that obliges councils, businesses and communities to recognise the sacrifice that individuals have given."
Mr Ellwood, who has rarely spoken about his actions, was praised by MPs in the week after the attack.
"I was one of many that stepped forward on that dark day," he told the Commons at the time.
At least 40 people were injured, including three other officers. Masood was shot dead by police.