A remorseful Martin Shkreli promised the judge who’s going to sentence him next week that he’ll be “more careful, open and honest” if she doesn’t impose a long prison term, acknowledging “I was a fool. I should have known better.”

“I assure you that any mercy shown at sentencing will be met with a strict adherence to this oath and I hope to make your honor proud of me in the years ahead,” Shkreli said in a letter penned from the Brooklyn lockup where he’s been since September.

It’s a turnaround for Shkreli, who was dubbed the most-hated-man in America after raising the price of a life-saving drug by 5,000 percent. He blasted members of a congressional panel who had quizzed him about the price hike, calling them “imbeciles” on Twitter.

“I am now, however a more self confident and secure person,” Shkreli wrote the judge. “The demons that haunted me — the root cause of my insecurity in my life — no longer all exist. I have learned a very painful lesson.”

U.S. District Judge Kiyo Matsumoto in Brooklyn concluded Monday that Shkreli caused investors to lose more than $10.4 million, rejecting his claim he made them money.

His lawyers said that decision means Shkreli could face a sentence of more than 30 years in prison, arguing it’s a term he doesn’t deserve. Noting sentencing guidelines are only advisory, defense lawyer Ben Brafman asked that Shkreli get 12 to 18 months and community service.

He is a “caring intellectual” who’s helped find cures for diseases that afflict kids, Brafman said of Shkreli. But he is plagued by “personal demons hell bent on self-destruction,” the lawyer added.

In Shkreli’s first direct communication with the judge, he called his five-week trial a “frightening wake-up call” and blamed his actions on insecurity, saying, “I wanted to be more than I was. I exaggerated.”

He acknowledged that he’d “dodged” questions posed by his investors or gave answers “that were only correct if put in a certain assumed context.” He described himself as an “irreverent and free-wheeling individual” whose comments and actions didn’t reflect his true nature.

“I regret where my temper can take me when I get angry or feel betrayed,” he said.

Matsumoto ordered Shkreli jailed in September after he issued a bounty on social media for a sample of former Secretary of State Hillary Clinton’s hair. Prosecutors have argued Shkreli should forfeit $7.4 million that he wouldn’t have had if he hadn’t committed the crimes. They are scheduled to make their sentencing recommendation on March 5.