Hillary Clinton faces “mounting health issues” — and she’s secretly worried that she’s too sick to run for president, according to a new book.

The 67-year-old Democratic front-runner has been “frequently plagued” by “blinding headaches” and a series of strokes over the course of the campaign which have left her second-guessing her chances of winning in 2016, says the upcoming book “Unlikeable — The Problem with Hillary.”

Excerpts were published Tuesday by Radar Online.

“For the first time I’ve known her, she’s showing self-doubt about her strength and vitality,” a friend of Clinton’s told author Edward Klein, who has written about the Obamas and Kennedys in the past.

The presidential candidate has also been battling bouts of insomnia that have ultimately “worried her, because it sapped her energy just when she needed it most for the campaign,” Klein writes.

“She is exhausted and depressed a lot of the time,” a friend is quoted as saying, adding that Clinton even tried taking sleeping pills to combat the problem, but to no avail.

“She said they made her less sharp the next day,” the friend explained.

In late 2012, Clinton suffered from a life-threatening blood clot on her brain that left her “constantly worried” she would develop another one, according to Klein.

Clinton has insisted she’s in good health and released a letter from her doctor over the summer attesting to her fitness.

Dr. Lisa Bardack of Mount Kisco, NY, outlined Clinton’s medical history, which included her treatment for a brain concussion, an analysis of blood clots affecting her legs and brain on separate occasions, an underactive thyroid gland examination, and a complete family history of heart disease.

Her findings showed that Clinton’s electrocardiogram and her blood lipids were reported as normal, and her cancer screening tests — including mammography, breast ultrasound, colonoscopy and gynecological examination — were also normal.

But Klein says the former first lady is keeping her medical ailments under wraps.

“There were incidents on the campaign trail when she felt faint and nearly swooned,” he claimed. “Those incidents were kept secret.”