Shaun Wilson-Miller, 17, recorded his message in his bedroom and intended to share it with his Facebook friends.
But now more than 1.3 million people have seen his heartbreaking home movie after a technical problem meant he had to upload it to YouTube.
Shaun, from Melbourne, Australia, suffers from a chronic heart condition that means his body has rejected two transplants.
Doctors said he cannot undergo a third operation - and it is believed Shaun has just months to live.
Shaun recorded the message in his bedroom late at night soon after hearing the devastating diagnosis.
He urged his loved ones to live their lives to the fullest and look after his dad when he is gone.
In the clip, a visibly emotional Shaun says: "I won't be here as long as I thought, but I wanted to say that this has been an awesome ride and I have no regrets.
"Live life to the fullest because you never know what's going to happen."
Since the emotional footage emerged, Shaun has appeared on a number of TV shows, including a guest role on Neighbours. He has met his favourite Aussie rules footballer Jobe Watson, is writing his memoirs and has inspired a rap song.
He has even found love with a fellow heart patient.
Shaun told a newspaper: "Yes, it's love. We've been going out for two and a half weeks, but we've been friends for seven years.
The hardest thing for me is leaving her, knowing that I won't get to marry her. To have kids together. To grow old together. That is what makes me sad.
His dad Cameron Miller said they have had to discuss his son's funeral arrangements because his new-found fame means the death will affect more than just their nearest and dearest.
"I just pray that it's quick. I don't want him to suffer," he said.
"No parent should have to bury their child. But he's loving life and he wants us to do the same. He doesn't want us to cry for him."
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