Van der Westhuizen revealed he had been handed a "death sentence" a month ago when the doctor who examined him told him he had three years to live.
"I'm fighting this illness with my faith. And my faith is strong," the 89-Test veteran told Rapport newspaper in South Africa.
The 1995 World Cup winner and the Springboks' joint-record try scorer has endured some difficult times during the last few years.
In 2009 he was admitted to hospital after suffering a suspected heart attack after watching a Test between South Africa and the British and Irish Lions, while his marriage came to an end in 2010.
He felt his life came tumbling down when he heard the latest news, and was most concerned for his two children.
"The first thing I did was to check if my [insurance] policies are in order for my kids," he said.
"The wind has been knocked out of my sails. I stared at the doctor in disbelief."
Van der Westhuizen realised something was wrong in December.
"I struggled to move my hand, but I just left it. It's typical; I thought it was an old sports injury," he said.
"But, as time passed my speech became impaired. When I spoke to people, they would say, 'Joost, are you drunk?"'
It was while on a break with his children in Sun City were playing with a doctor friend in the pool when he realised there was something wrong with his arm. He talked about his problem, and his impaired speech and the doctor immediately sent him to a neurologist.
His doctor received the results and Van der Westhuizen said it was the first time he saw his doctor - also a close friend - being emotional as he told him the outcome.
"After he told me I have motor neurone disease, he said that doctors didn't know how to treat it," he said.
"I became a blank. I hardly remember anything from that day."
A second opinion has been sought from a Johannesburg neurologist but he has still to hear her assessment.
He said he had great support from his parents and brothers.
"I will fight this thing. Even if it's the last thing I do," he said.
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