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Jon Gosselin and his ed girlfriend Hailey Glassman took a romantic carriage ride through Central Park in October during happier times.
"He became a monster," she told RadarOnline.com. "He's a master manipulator and a compulsive liar."
Glassman, 22, said she learned the gut-wrenching truth about her eight-month relationship to the former "Jon & Kate Plus 8" star during a deposition on Monday when the details of Jon's relationship with tabloid reporter Kate Major were revealed.
"I was told under oath that Jon really did sleep with Kate Major," Glassman said. "I'm mind-boggled and hurt by it. And he still lies to me about it and denies it to this day."
Glassman, 22, said she had been duped into thinking the blonde ex-Star magazine reporter felt bad for her after Gosselin -- who is being sued by TLC for breach of contract -- said he had a "friend" who wanted to do a "defending Hailey" article.
"I think back now and say to myself, 'Hailey, you should have been smarter.' I mean they told me, 'Kate Major is going to pretend to be Jon's girlfriend so they can take the heat off of you,' and I was like, 'Great! Ok, thank you!'" Glassman said.
But when photos surfaced showing the father of eight and Major hobnobbing in the Hamptons and gallivanting around Manhattan, Glassman said she felt it was the "ultimate betrayal."
Glassman is the daughter of the surgeon who performed a tummy tuck on Jon's soon-to-be-ex-wife Kate. But now she blames the divorcing dad for causing a rift in her close-knit family.
"Welcome to my nightmare," Glassman told RadarOnline. "I'm not on speaking terms with my mother because of Jon. He's thrown a huge fork in my family. That's the saddest part about all of this."
"I just want this all to be over," she said. "I want him out of my life. I think Jon needs to relearn to love himself before he can even have friendships. He needs to relearn how to be a good person, an I hope he gets help."