Crystal Hefner was just 21 years old when she met Hugh Hefner, 81, at the Playboy mansion’s infamous Halloween party, which was known for its guest list of celebrities and scantily clad models.

He asked her to move in with him — and his 18-year-old twin girlfriends, Karissa and Kristina Shannon — days later.

“It was very, very fast. I think he had a lot of experience with just moving people in right away,” Crystal told The Post. “The ‘I love yous’ started pretty quick.

“I’ve learned all about love bombing since then.”

It led to marriage in 2012, when Crystal was 26. Over the 10-year relationship, she buried who she really was, lived with a 6 p.m. curfew, couldn’t travel and gave herself over to a “needy” man who she felt too guilty to leave.

She was at Hefner’s bedside when he died in 2017, at age 91.

But it’s only been recently that Crystal, now 37, has come to terms with the darker side of her life with Hef.

It’s taken hours of therapy and “deprogramming” to untangle the debris of her life with the man who founded the Playboy empire in 1953. She’s thrown away all but one of her “bunny girl” outfits, removed her breast implants and spent the past year-and-a-half writing a memoir called “Only Say Good Things.”

“It’s called ‘Only Say Good Things’ because I [had] a conversation with Hef and he let me know: ‘Once I go, when I’m gone, please only say good things about me,'” Crystal told The Post.

“I kept that promise for the last five years. After going through a lot of therapy and healing, I realized that I needed to be honest about my time there. The book is about healing from a toxic environment.”

Born Crystal Harris in Lake Havasu, Arizona, she soon moved to Birmingham, England — where her family lived upstairs from her parents’ pub — before settling down in San Diego at age 8.

She was a psychology student at San Diego State University and a part-time model when a friend suggested she submit a photo to attend a Playboy party.

Describing herself as “insecure and young,” Crystal was surprised when she was invited to the 2008 Halloween bash. There, she struck up a conversation with Hef and they found common ground: he had studied psychology at the University of Illinois 40 years before she was born.

Crystal recalled being “enthralled” by the “surreal scene,” describing it as Willy Wonka like and revealing: “It’s like, ‘Oh, this is how the other half live.'”

Just a day after she arrived home, Hef called to invite her to move in and be his girlfriend along with the Shannon twins.

At first, Crystal said, she thought, “Wow, I finally belong somewhere and I’m important — by association, but I’m still important. Yeah, it feels good.”

It didn’t take long, though, before “the facade [and] everything kind of unraveled … Everyone was kissing an 80-year-old.”

In the recent A&E docu-series “Secrets of Playboy,” Karissa compared sex with Hefner to rape and said she had an abortion after becoming pregnant by him at age 19.

“I felt disgusted with my body,” Karissa, now 33, said.

Crystal recalled Hef’s relationship with the young twins as “so strange”: “I remember Hef holding them, like, ‘My babies.’ [It was] so weird. They were 18.”

Crystal soon found herself in a coercive relationship.

“As he got older, [Hefner] just got more needy and dependent on me,” she said.

“Hef loved the old movies where the women were just fainting and helpless, and they could do nothing without a man, and they asked a man for everything,” she said, adding that she was “rewarded for being the helpless damsel.”

And it was all about the business, all the time, as Hef would constantly ask Crystal to “wear the flag” —  the Playboy bunny logo. It led to competition among the women.

“The outfits for the parties, you had to try and look your best so that you’re looking as good or better than the other girlfriends,” Crystal said.

“You’re in a place where you could easily be replaced, so you’re always kind of on guard,” she added. “It’s hard to make friends.”

She also had to live by the house rules — including a 6 p.m. curfew.

“When Hef would go out, it would be to a club to go pick up women. So I think he assumed that, when I was going out, I was going to go do the same thing, Crystal said .Really, I just wanted to go to Disneyland or to the beach, or to anywhere.

“One time, I asked him to travel to Paris because I wanted to go Paris Disneyland … He said, ‘No, go when I’m gone.'”

She stopped asking.

Crystal and Hef were set to get married in June 2011 but she called it off five days before the wedding. They eventually wed in December 2012 — in part because of guilt she felt.

“Towards the end of his life, I felt like I couldn’t leave him. I had to take care of him,” Crystal said. “It was like, ‘OK, he adores me and he needs me, and he leans on me for so many different reasons and I can’t leave him.’ So I was there till the very end.”

Reflecting on their time together, she added: “I’m only now just kind of learning what it means to be in a healthy relationship. 

In 2015, Holly Madison — Hefner’s previous ‘No. 1’ girlfriend, who had starred on the E! reality show “The Girls Next Door” alongside his other main girlfriends at the time, Kendra Wilkinson and Bridget Marquardt — had claimed that that Hefner had “all kinds of naked pictures” of women to allegedly use for blackmail.

Crystal thinks that Hefner took the photos for fun and had no intention of blackmail. But after his death, she said, “I cleaned the house out and I found them. I just thought about if I were the women in these photos, what would I want to have done with them? So I made sure to rip them all up. They’re all shredded and thrown away.”

After Hefner died on Sept. 27, 2017, “I couldn’t even leave the house or my bedroom for two weeks because I was so upset,” Crystal recalled.

“Eventually my friend invited me away … my very first trip outside the mansion was to Africa, and we kept that trip going. We went over to Australia. It was just a month-long trip. It was very needed.”

She’s now teamed up with a friend for this week’s launch of First Ape Wives Club — “a collection of 10,000 NFTs that will serve as your digital membership pass … [to] luxury travel amenities like complimentary upgrades,” created with women in mind, according to its Twitter, as well as members-only parties and dedicated travel-agent support.

“I’m learning wh