Just two weeks after the birth of his daughter, Jack Osbourne, 26, found out he has multiple sclerosis. Jack, his mom, Sharon, and his fiancée, Lisa Stelly, open up in an exclusive interview in this week's issue of PEOPLE about how he's coping with the disease, the fight to save his vision, and his future. "I never felt sorry for myself," Ozzy & Sharon's son tells PEOPLE. "But it was a bit of a buzzkill, that's for sure… It's just one of those things you take as it comes…"

With a "perfect" 2-week-old baby girl Pearl Clementine at home, a wedding in the works, and a flourishing career as a TV producer, Jack Osbourne was on top of the world – despite having woken up with a headache – when he stopped into an L.A. gas station for a soda one night in May. Then everything changed. "I was talking to the attendant, and all of a sudden a black dot appeared in my vision," Jack tells PEOPLE. "The next day I woke up and the dot turned into a cigar shape." He shrugged it off, thinking his contact lenses caused the problem. But "after three days I knew it was bad. I couldn't see anything in front of me with my right eye – all I could see was my peripheral vision. My central vision was gone."

Jack finally went to the doctor, where a barrage of tests revealed a lesion on the brain. The diagnosis – multiple sclerosis – came as a shocking blow to Jack, his fiancée Lisa, 25, and his tight-knit rock and roll clan who became a household name a decade ago with their MTV reality show: dad, Ozzy, mom Sharon, and sisters Kelly and Aimee. "The first thing Ozzy and I asked ourselves was, 'What did we do wrong?'" Sharon tells PEOPLE in a chat in their West Hollywood condo. "Was it something I did when I was pregnant? You question yourself. It's not meant to be at 26." Kelly says she felt "heartbroken and helpless." Jack's reaction to his diagnosis was anger. "I was pissed off because so many good things in my life were happening," he says. "I think when God came to review my file, he was like, 'Nope, too much good. Make bad.'"

Jack adds, "My mom and dad took it worse than I did. They just knew how happy I was with Pearl and Lisa. They were heartbroken." When he left the hospital for his L.A. home, he cried. "I had a soppy day and a half," he says. "I kept thinking, 'Why now?' But I couldn't sit there feeling sorry for myself. I have a family to think about." He and Lisa are still planning an "intimate" fall wedding, he says. "The good thing about having a little baby is that I can't sit around and think about myself all day," he says. "Lisa and I are too busy changing poopy diapers and trying to figure out why she's crying at 3 a.m.!"

Jack plans to begin a "pretty insane" regimen of vitamin supplements and hormone replacement therapy to make his body stronger. "Right now am I going to be in a wheelchair? No. But if I don't take care of myself, who knows," he says. Adds Lisa: "This could end up being a huge blessing in disguise – cause him to change his life drastically to be stress-free and change his diet." Symptoms could flare up as seldom as every several years, though they usually grow more frequent and debilitating as a patient gets older. "There's no rhyme or reason. That's the worst part….," Jack says. Still, "I think within the next 20 years they're going to have a cure for this."

The full interview with the Osbourne family appears in the 7/2/12 issue of PEOPLE, on newsstands now.

*Also In This Week's 'People'*
Exclusive: John Edwards' Mistress, Rielle Hunter, Tells 'People': "We're Still A Couple"
Michelle Obama & Ann Romney: The Last Book They Loved