"grow unbearable" is correct and usable in written English.
You can use it to describe a situation that becomes increasingly difficult or unpleasant to the point of being unbearable. For example, "The heat in the room began to grow unbearable, so everyone moved outside."
Her life was also shaped by the medical problems that grew unbearable in her later years.
Mrs. Sejdaras hated being confused, so she avoided taking pain medication until her pain grew unbearable.
The next day, after a hike through the tall grass, the stench grew unbearable and the air reverberated with the sizzle of thousands of flies.
One night, when the pain grew unbearable, she called for the nurse, who shone a torch into a puncture wound in her head.
The evening before, after miles of hard pedaling, he was forced to forfeit a fare when his stomach pain grew unbearable.
The travel, which was always hard on Ochoa, grew unbearable after her December marriage to Andrés Conesa, the chief executive of Aeroméxico, who had three children with his first wife.
Her pain was growing unbearable, but she had finally come to the end of her book, and she was determined to type out a fair copy to show to publishers and agents when she went back to England for Christmas.
The girls' parents treated them at home but then Sneha's temperature rose to 104 degrees, a rash spread across her legs and shoulders, and her pain grew unbearable.
By now, he had only an indistinct sense, in another layer of his mind, that long, long ago someone had thrown him fresh meat twice a day and sprayed him with water when the heat grew unbearable.
From childhood on, books held a place of almost religious prominence in Wharton's life.In this section The squeeze is on Dazed and confused How not to win friends Grand dame The rage of giving Poland's loss ReprintsA passionless relationship with Teddy Wharton, whom she married at 23, grew unbearable when his mental health collapsed.
