His body was found by a member of the public 10 days later in the water near Chidley Point Reserve in Mosman Park, where police had been searching since last week. Police say there are no suspicious circumstances.
Ms Roulston told the Rotorua Daily Post through tears it had been an agonising wait.
"It's horrendous. I would not wish this on anyone. They say the pain of not knowing is worse. I kept trying to make up a scenario of what might have happened... you do anything to keep them still alive."
But she is grateful to have closure and his body has been found. "I am so broken."
Mr Mitchell was born and raised in Auckland but moved to Rotorua in his teens where he went to Rotorua Boys' High School. He left Rotorua after secondary school, living in Waihi, Wellington and with his father in Perth, before returning to Rotorua five years ago to live with his mother when her partner died.
She said the two of them were very close and she appreciated him coming home to look after her.
He stayed with her for a couple of years in their Mamaku home before moving back to Perth to start his business, The Mining Hub, a mining contracting business.
"I just can't understand why he would get in a dinghy. He was so safety conscious at work and would drive his employers mad."
When he lived in Perth, he would often ring his mum and ask her to talk to him on the phone to help send him to sleep.
Ms Roulston's face lit up when she talked about how much of a "joker" her son was.
"He had the gift of the gab and would draw people in. He was a right prankster and would play tricks all the time."
On her birthday, he would always leave it to one minute before midnight before ringing her to say "happy birthday", she said.
Family friend Kim Stuart, who was helping look after Ms Roulston, said she had known Mr Mitchell all his life. He was the type of guy who loved the limelight and a small part of him would have loved the attention his death brought, she said.
"I guess he was the type of guy who was going to go out with a bang... he'd be saying 'I was on the news and everything'."
Ms Stuart and Ms Roulston said he would probably make a joke out of the fact he had died before his mother.
"He was that sort of guy, he's probably saying right now 'I beat her to it'," Ms Stuart said.
Ms Roulston has stage four lung cancer, frequently uses a nebuliser to help her breathe and is on morphine for the pain.
I guess he was the type of guy who was going to go out with a bang... he'd be saying 'I was on the news and everything'.
Her doctor told her she would have between two and six months to live and that was five months ago.
"So I'm doing pretty good."
On the night her son went missing, Ms Roulston was staying with a friend in Kawerau and couldn't work out why she was having a restless night - putting it down to her failing health.
"I couldn't settle. I had to keep getting up and down that night. Now I know."
Ms Roulston said she was staying in contact with her son's father in Perth who was making funeral arrangements, but she would be too sick to attend.
She was hoping some of his ashes might come back to Rotorua to be with her.
She also hoped his friends in Rotorua who aren't able to make the funeral could do her a favour.
"Can they listen to Nights in White Satin and think of him during that song, it's a special song."
