When the tour wrapped earlier this month, Gilbert met with a neurosurgeon in Los Angeles. "He came in and said, 'Your back is broken, and it has been for months,' " Gilbert, 46, reveals to PEOPLE. "I sort of was in shock for a while."
Gilbert, who played Laura on the original Little House TV show and Ma in the musical, fought through the pain for the sake of the production. "I would balk and fight it and say, 'I've got to do the show,' " she recalls, telling herself: "If it got really bad and I lost feeling in my legs or couldn't move, I'd go to the ER."
She adds: "I thought doing a musical at 46 playing a woman in her late 20s, early 30s was amazing enough. But doing it with a broken back buys me a little more street cred."
The actress will undergo surgery on July 22 to replace a disc with a plastic implant and fuse a vertebra in her lower spine. "One of the things that is great is that I've lost about a half inch in my height and I will get that back," she says. "That's really cool."
A complete recovery will take six months, but Gilbert is planning to return to work at the end of September to shoot a film. "I won't be jumping around doing a musical for a while," she says. "The recovery is intense, but my husband [Bruce Boxleitner], my kids and my friends, the casserole brigade, they're ready."
A self-described "sober alcoholic," Gilbert has remained diligent when it comes to pain meds. "I'm really careful about this stuff," she says. "I don't think you'll be seeing me getting arrested for driving under the influence of Percocet anytime soon."