Greg Wise believes therapy "saved [his] life".
The 55-year-old producer-and-actor - felt "fortunate" to be able to access professional help because he thinks it ultimately helped him to "survive" following the tragic loss of his sister Clare, who died of bone cancer in 2016 aged 51.
Speaking in his role as Marie Curie ambassador, he said: "I think the best work I have ever done in my life has been therapy. It's absolutely saved my life.
"I was very fortunate that I had taken myself off and done an awful lot of work on my head before the time that I had to go and be with my sister. And I think that's the reason, one of the reasons, I was able to survive that."
The 'Effie Gray' actor found caring for his sister during her illness to be a "privilege".
He said: "As was clear then and has sort of become even more clear, is the trauma of it is absolutely balanced by the privilege of being able to be there, and to be the carer and to bear witness and to shadow someone you love walking this terrible journey.
"One of the things about that traumatic and privileged time is, when I was able to emotionally calm a bit afterwards, I realised that I was formed into a better person from having done it.
"We all talk about running into the flaming building to rescue the child, and we think, 'Yes, I would absolutely do that.'
"But that's always a hypothetical until you're confronted with a building on fire with a child inside. And those moments I spent with my sister, I knew that I was capable of, in that respect, going into the burning building."
Greg - who is married to Dame Emma Thompson - was encouraged to live "a little bit better" and to push himself with new challenges following the death of his sister.
He said: "To live a proper life, you have to understand that it's finite.'
"One of the things with my sis, because she died when she was 51, which is still really young, it made me realise that we have to try and find a way of just living a little bit better."
The 'Sense and Sensibility' actor was motivated to take part in 'Strictly Come Dancing' last year by the memory of his "disco diva" sister.
He said: "I was driven primarily by my sister into it because she was the big disco diva... she was cremated in a glitterball coffin to banging disco music.
"And she would have been thrilled that it was absolutely that, it was seeing what was possible, and if it was possible for me to go on a live evening show and do my American smooth or whatever, before the car crash of the samba!"