"She got me through my darkest times," Kardashian told People in a new interview about his fiancee. "I wasn't happy in my own skin. I missed out on so many things."
But Chyna, 28, whose real name is Angela White, managed to get through to him.
"I've been friends with Chyna for a long time," Kardashian explained. "She was someone I reached out to for advice. She would cook for me and we'd just talk. She's surrounded me with a lot of positivity. From the moment we met, I knew I wanted to be more than friends."
For her part, Chyna just wanted to provide support.
"I saw Rob was willing to better himself," she told People. "I had never wanted to be tough. I never wanted to change him."
Kardashian's slide into seclusion started in 2014, when he skipped sister Kim Kardashian West's wedding to Kanye West.
"I was doing my suit fittings in Paris right before the wedding and I just wasn't comfortable," Kardashian revealed to People. "I'm 6'1" and at my most I probably weighed 300 lbs."
"There were cameras at the airport on our trip there and I was very unhappy with the person I saw in all the pictures," he explained. "I'm upset I missed my sister's wedding but it was a personal decision."
When he returned back home, Kardashian retreated even further, skipping "Christmases and birthdays and vacations" and staying outside the spotlight for nearly two years.
All that changed when Chyna entered the pictured.
"Chyna definitely motivates me," Kardashian said. "Being with her got me over the insecurity of being out in public."
Now the pair is expecting their first child together later this year and have a new E! reality show, "Rob and Chyna," that premieres Sept. 11.
"We just want to live our lives the way we want to," Kardashian said. "There were definitely dark days for me, and I'm still not fully happy looking at myself in the mirror. But I'm working towards that. And I'm happy."