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

Karen Salmansohn says she 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."

ASUNDER: Mitchell Leff and Karen Salmansohn.

But when the $28,000 treatment Leff paid for was successful, the Laundromat king told her he was "too old" to be a dad again and she should get an abortion, her Manhattan Supreme Court lawsuit alleges.

Now she's suing to make him support her through the pregnancy.

Leff, 51, declined to comment.

The pair met on Aug. 24 "and commenced a romantic relationship," the suit says. Leff told Salmansohn their divorce would soon be official.

She told him on their second date she was looking to settle down, the suit says.

Leff said he'd be "very happy" to start another family, and weeks later popped the question. He also volunteered to pay for pricey IVF treatments.

By November, Leff offered to pay for renovations on her Chelsea apartment to make it more "family friendly."

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.