"I went to 17 doctors, and nobody could figure out what was going on," the actress, best known for her turns in 3rd Rock from the Sun and TV Land's The Exes, tells PEOPLE. "I didn't know what was going to happen to me."
After months of painful tests, endless doctor visits and taking leave from work, Johnston, 46, finally visited rheumatologist Dr. Daniel J. Wallace, who diagnosed her with lupus myelitis, a rare form of lupus that affects the spinal cord. By then, the autoimmune disease had left her so weak and exhausted she couldn't lift her neck without a brace.
"If I had to live like that forever, I would have killed myself," she says.
Upon her diagnosis, the actress checked into the hospital for treatment, including chemotherapy and steroids. Six months later, "she's in remission," Wallace tells PEOPLE. "But if she'd waited a few weeks longer, she could have ended up quadriplegic."
Though she's still recovering – Johnston suffered a shattered bone during a hike recently, due to side effects of her treatment – the actress says, "Every day I'm amazed that I can lift my head."
"Every single day is a gift, and I don't take one second of it for granted."
For more on Johnston's battle with lupus, pick up the latest issue of PEOPLE, on newsstands now
Reporting by CHARLOTTE TRIGGS with EMILY STROHM