Cassie Ventura revealed excruciating new details Tuesday about Sean “Diddy” Combs’ twisted “freak-off’’ sex sessions — including how they could last for more than four days and quickly “became a job.”

“Within the first year of our relationship, Sean proposed to me this idea, this sexual encounter he called voyeurism, where he would watch me… [have] sexual activity with another person, specifically another man,’’ the singer told the jury at Combs’ sex-trafficking trial in Manhattan federal court.

“I just remember my stomach falling to my butt,’’ said Ventura, now 38, married to Combs’ former personal trainer and eight months pregnant with their third child.

“I think I was 22 at the time. … I didn’t have a concept how that would be a turn-on. But I felt a sense of responsibility since he was sharing something like that with me,” she said.

“I was confused, nervous, but I also loved him very much, and I wanted to make him happy.”

The sex encounters usually involved “the hiring of an escort and setting up this experience so that I could perform for Sean,’’ she said.

“It just involved watching Sean be able to watch me with the other person and actually direct, direct us with what we were doing,’’ Ventura said.

Ventura said she didn’t know how the now-infamous term came about.

But “pretty quickly over time, I knew it wasn’t something I wanted to be doing, especially as regularly as it became,’’ she said.

“Eventually, it became a job for me pretty much. … It was something he would want to do. I would have the contact to set it up. … But in the beginning, Sean set it up.”

Venture revealed how it could take “all of that day, time” to get prepared physically for a “freak-off,” with Combs demanding to know whether she had gotten waxed, tanned up and her nails done.

She described how she couldn’t even fully concentrate on her music career because of what it took to perform up to Combs’ satisfaction for his creepy, voyeuristic events — which occurred everywhere from New York City, Miami, Las Vegas and Atlanta to Ibiza, Spain and Turks and Caicos.

“When I wasn’t working on my music, I was recovering from partying — that took a big chunk out of my life,’’ Ventura said.

“The freak-offs became a job where there was no space to do anything else but to recover and try to feel normal again.

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“Staying up for days on end, taking drugs, other substances. Drinking. Having sex with a stranger.

“Thirty-six, 48, 78 hours. The longest ever was four days, maybe even more, on and off with the breaks. It was significant,’’ she said.

“I was recovering from the drugs, dehydration, just staying awake all that time.”

“He would bring them up at random times,’’ Ventura said of the “freak-offs,’’ which involved other women having sex with men in front of Combs at his command, too — with plenty of baby oil on hand.

“I didn’t know what one would be or what one would turn into.’’

She said she continued agreeing to them partly because “again I was in love, and I just wanted to make him happy.”

She said she was also concerned about making Combs angry — with some of their arguments eventually becoming extremely violent.

“He would back me on my head, knock me over, drag me, kick me, stomp me on the head,’’ Ventura testified.

Asked how often the violence occurred, she replied, “Too frequently.

“Ultimately at that point, Sean controlled a lot of my life. Whether it was career or the way I dress, like everything, everything. And I just didn’t feel like I had much say into it at that time, being super young, naive, a total people pleaser. I didn’t know enough if he would be upset to be violent or write me off,” she said.

Ventura met Diddy at around 19, when she was modeling in New York City, and soon signed a 10-album record deal with his Bad Boy Records label in 2006.

Combs has pleaded not guilty in the case.

Additional reporting by Ben Kochman and Priscilla DeGregory