In the anguished letter to Florence Campbell, the young digger described lying in muddy holes for days during battles on the Western Front.
Having fought at Gallipoli, he recalled the debilitating effects of mustard gas and illness in the trenches, as men died around him.
When his extended family read the missive many years later, Private Jensen's admission of mental torment was one of the starkest details.
"I must be left alone. I cannot get married and look after you when I shall be scarcely able to look after myself," he wrote.
More than a century later, that letter inspired Pte Jensen's great-nephew, Pieter Lindhout, to walk 400 kilometres in the footsteps of other World War I soldiers to raise money for youth mental health.
Mr Lindhout walked from Gilgandra, in western NSW, to Sydney last year, re-enacting the journey of 35 men who left their lives behind and marched to the city to enlist in October 1915.
It was known as the Coo-ee March, one of many recruitment marches in regional NSW and Queensland, a grassroots movement to boost enlistment after the loss of thousands of men at Gallipoli.
The Coo-ee men gathered more than 200 extra recruits as they passed through towns on their way to the city.
The legend of the marches lives on through descendants and locals, who stage re-enactments, care for monuments in their honour and remember them every Anzac Day.
Mr Lindhout felt a deep connection to the Coo-ee story, having spent years poring over his great-uncle's letters and turning them into a book, Love in the Valley of Death.
Like many of the men in Gilgandra, Pte Jensen had also grown up in rural Australia and went to war with an initial sense of adventure.
Mr Lindhout said his own trek gave him time to reflect on the Coo-ees' long walk to Sydney followed by months on a boat and years at war.
"That level of sacrifice is extraordinary. And I don't think we really even understand that sacrifice today," he said.
Gilgandra historian, Margo Piggott, has helped bring to life the stories of the Coo-ee March for the town's museum and historical society.
She says young and middle-aged men, farmers, bricklayers, bakers, a journalist and a postman, went to war, fuelled by a combination of adventure, patriotism and decent pay.
Many were badly injured or killed. A statue in Gilgandra honours their "eternal footprint on the history of our community and our nation".
Numerous Gilgandra families are dedicated to remembering and honouring the Coo-ee men and re-enacted the march in 1987 and 2015.
Ms Piggott says the stories of mateship from the Coo-ees and the service men and women who followed, have informed the way Anzac Day is commemorated.
"It's about helping your mate and making sure you look after each other," she said.
"There's more awareness of the impact and what they're like when they come home."
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