The 29-year-old's battle with depression led to weight gain and Rob ballooned to almost 300 pounds (136 kilograms) in 2014, while his big sister was planning her European wedding.
Embarrassed by his size, he decided he wasn't able to face the public and opted not to attend the nuptials in Italy, even though he was in Paris for the rehearsal dinner.
"I was doing my suit fittings in Paris right before the wedding and I just wasn't comfortable," he tells People.com in a rare interview. "I'm six foot one (inch) and at my most I probably weighed 300 pounds."
"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 continues. "I'm upset I missed my sister's wedding but it was a personal decision."
Rob returned home after the wedding and became a virtual recluse, skipping family holidays and vacations.
"I just wasn't down to have photographers follow me, so I made sure I was away from all of that," he says. "I'm very good at disappearing."
"I wasn't happy with myself so I didn't want my family to see me that way," he adds.
His decision to stay away from his family members led to a showdown with sister Khloe, who has battled her own weight issues over the past decade.
With a new positive outlook on life after becoming engaged to model Blac Chyna in April (16), and a child on the way, Rob admits he's in a good place and he's determined to stay on top of his feelings and emotions.
"The way I dealt with my issues maybe wasn't the best, but nobody can make me happy but me," he says. "Now I want to see everybody in my family every chance I get. You never know what is going to happen tomorrow, and I don't want to do that to my mom.
"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."