The author of the self-help book the “Prince Harming Syndrome” is fighting back against the cad she says royally hosed her.

Karen Salmansohn, the writer of “How to Make Your Man Behave in 21 Days or Less Using the Secrets of Successful Dog Trainers,” says she was was knocked up by a Long Island man who’d falsely promised to have a family with her.

Mitchell Leff told Salmansohn, 49, he was “practically divorced” when he met the “How to Succeed in Business without a Penis” writer and wanted to start a family with her – and even offered to pay and provide sperm for IVF treatments, Salmansohn says in court papers.

“I’m a self-help author, not a psychic,” Salmansohn told The Post. “His words and actions were completely telling me he wanted a child.”

But when the $28,000 treatment Leff paid and provided for was successful, the 51-year-old landromat king told her he was “too old” to be a dad again and she should get an abortion, her Manhattan Supreme Court lawsuit charges. Now she’s suing to make him support her through the pregnancy as she says he’d promised to do.

Leff declined comment.

The pair met while having breakfast at Pastis on Aug. 24 of last year “and commenced a romantic relationship,” the suit says. Leff and his wife had been separated for two years, and he told her their divorce would soon be official.

The “Even God is Single So Stop Giving Me a Hard Time” writer told him she was looking to settle down and fast on their second date, since “at her age, starting a family was a top priority,” the suit says.

“It was no pressure. I was just being upfront, seeing if he would be open to it, because if not for me it could be a dealbreaker,” Salmansohn said. “I’m all about communication and saying what my dealbreakers are. It’s something all women should do. I practice what I preach.”

Leff said he’d be “very happy” to start another family, and quickly put his money where his mouth was, popping the question weeks later with a $10,000 Tiffany’s engagement ring and volunteering to pay for pricey IVF treatments straighway. The couple met each other’s families, and told them they were engaged.

“There were no red flags. None,” Salmansohn said.

By November, they’d made ten trips to the fertility clinic together, the suit says, and Leff offered to help her out monetarily and pay for renovations on her Chelsea apartment to make it more “family friendly.” Over a six month period, the suit says, Leff “spent lavishly” on Salmansohn, shelling out over $150,000 on vacations, renovations and other assorted odds and ends. “He pursued me very enthusiastically,” she said.

Leff also promised to pay all of her medical expenses and everything needed to prepare the baby’s room, and said he’d support her in excess of the $150,000 she typically made during the year while she was pregnant and taking care of the baby.

He had a sudden change of heart in December after learning the IVF had been successful. A “mere 24 hours after learning of the pregnancy,” she says he told her “I already raised two kids. I don’t want to raise any more,” and “advised” her to abort the pregnancy.

Salmansohn, “now pregnant with the potential for her first child at age 49, refused to consider abortion,” the suit says.

Leff, who’d encouraged to Salmansohn to pass on jobs while they were still involved, supported her financially for a few weeks, but cut her off last month.

Leff “abandoned plaintiff, without any prospects for income, while she was in a state of physical and emotional challenges due to pregnancy, with the additional emotionally traumatic burden of being abandoned by the man she loved, who she thought loved her, while she was pregnant with the child he’d enthusiastically deliberately created,” the suit says.

And while they’d lived in different homes for two years, Leff also acknowledged that he’d yet to file for divorce from his estranged wife – and that he’d made much of his cash by working as a bookie for the last 30 years.

The suit seeks to force Leff to make good on his “oral contract” with the guru.