Prince William says becoming a parent brought back painful memories of Princess Diana's death

The Duke of Cambridge said he found becoming a father 'overwhelming' at times. (Getty Images)

Prince William was thanked by the Archbishop of Canterbury on Sunday for encouraging him to seek help for depression.

Now the Duke of Cambridge, 37, opens up this week in a new documentary about his own mental health.

The father-of-three has revealed that becoming a parent brought back painful memories of Princess Diana’s death.

Speaking in BBC One’s Football, Prince William And Our Mental Health, which will be aired on Thursday evening, he admitted finding things "overwhelming" as the “traumatic” event from his teenage years resurfaced.

The royal father-of-three was 15 when he lost his mum (pictured with her sons in 1995). (Getty Images)

Read more: Prince William encouraged Archbishop of Canterbury to seek help for depression

The duke was 15 years old when his mum passed away following a car crash in Paris in 1997.

For the upcoming programme, former professional footballer Marvin Sordell told him that becoming a dad was "the hardest time in my life", having not grown up with a father figure.

The royal replied: “Having children is the biggest life-changing moment, it really is.

"And I agree with you, I think when you've been through something traumatic in life – and that is like you say your dad not being around, my mother dying when I was younger – your emotions come back in leaps and bounds because it's a very different phase of life.

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"And there's no one there to, kind of, help you, and I definitely found it very, at times, overwhelming."

The duke acknowledged that emotions can "come out of the blue".

He added: “It's one of the most amazing moments of life but it's also one of the scariest."

Along with Kate Middleton, 38, he is father to son Prince George, six, daughter Princess Charlotte, five, and two-year-old son Prince Louis.

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The duke has previously opened up about how his mental health was affected by his mum’s death.

In another BBC documentary, broadcast last year, he said: “I've thought about this a lot, and I'm trying to understand why I feel like I do.

“But I think when you are bereaved at a very young age, any time really, but particularly at a young age, I can resonate closely to that, you feel pain like no other pain.”

He also noted that his former job as an air ambulance pilot “tipped him over the edge”, partly because he worked “several times on traumatic jobs involving children”.

The duke said having his own family caused him to feel “very sad and very down”.