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Glen Campbell knew something wasn't right. A year ago, when recording his upcoming album Ghost on the Canvas, the Country Music Hall of Famer sometimes found himself unable to remember his own lyrics or a once-familiar lick on the guitar-the instrument he'd played for a lifetime. "Glen would get confused and frustrated: 'Why can't I sing this? Why can't I play this?'" recalls producer Julian Raymond. "He'd ask, 'Is my mind going?'"

During a chat with the affable Campbell, 75, on a recent afternoon, the answer to his question is poignantly apparent. Over the course of two hours, he repeats himself frequently, struggles to recall details of his life and is unsure of his age or even what he had been doing earlier in the day. He also doesn't remember that six months ago he was diagnosed with Alzheimer's disease.

"I have Alzheimer's?" he asks Kim, 53, his wife of 29 years, while they drink coffee together in the kitchen of their sprawling Malibu home. "Well, doggone ... what's that?" Kim gently reminds him that it's the reason he's been having trouble remembering things, but Campbell prefers a different explanation for what's happening in his brain. "God just cleared a lot of things out," he says cheerfully. "It was crowded up there. I've been trying to get rid of that crap for years."

The Campbells have decided to go public with Glen's diagnosis because he's hoping to say farewell with a final slate of live performances this fall-and they want his fans to be aware of his condition. "Glen is still an awesome guitar player and singer," says Kim. "But if he flubs a lyric or gets confused onstage, I wouldn't want people to think, 'What's the matter with him? Is he drunk?'" In fact Campbell's neurologist Dr. Hart Cohen finds his resilience "inspirational," noting, "Glen loves to perform, and that has not been hampered by this disease."

It's what he's done best for over half a century. The seventh son born to a sharecropper in Delight, Ark., Campbell became one of the recording industry's premier session musicians while in his early 20s, backing everyone from Elvis Presley ("That's my guitar on 'Viva Las Vegas,'" he says proudly) to Frank Sinatra to the Beach Boys. He can still tell stories of his meteoric rise in the late 1960s to the top of the charts as one of country music's first crossover artists, with hits like "Wichita Lineman" and "Rhinestone Cowboy." He remembers costarring with John Wayne in the original True Grit and hosting four seasons of TV's Glen Campbell Goodtime Hour. In conversation he loves to break into a cappella snippets of beloved "hillbilly" songs from his boyhood.

There are, however, parts of life Campbell might prefer to forget, particularly his bouts of cocaine and alcohol abuse. Though he'd sworn off drugs and alcohol at Kim's insistence upon their 1982 marriage-"He'd be mean, and it looked like the devil was in his eyes," she says-Campbell fell off the wagon in 2000. The singer was arrested in 2003 on drunk-driving charges, resulting in the release of an embarrassing mug shot and a 10-day jail sentence. "Glen checked into the Betty Ford Center," says Kim. "The doctors asked him a lot of questions, and he couldn't answer them. That's when we first realized there was a problem."

Campbell was diagnosed with "mild cognitive impairment," and his condition stabilized for a number of years. But in 2010 "there was a profound deterioration in his memory and his cognitive skills," says Dr. Cohen, an attending neurologist at L.A.'s Cedars-Sinai Medical Center. That led to the diagnosis of Alzheimer's, which Dr. Cohen says is still in a mild stage: "I think and hope he can remain fairly functional and social for at least several years."

Kim, a former Radio City Music Hall Rockette who first met Campbell on a blind date in 1981, says some days her husband "almost seems normal," but on other days confusion sets in. "Glen will ask, 'Where's the bathroom?' in the home he's been living in for six years," she explains. (Cracks Glen: "I do that a lot just to bug her.") "He'll ask me if I'd like a bite of a carrot and he'll be holding a banana," she continues. "He's gotten lost several times driving his car and finally had to give up his license."

Still, she is emphatic that there is nothing exploitative in Campbell's return to the road. "I hope people don't think that," she says. "I hope people realize it's fulfilling for Glen to do music and feel all the love he gets from his fans. It really keeps him going." A teleprompter will help him remember lyrics (sample line from "A Better Place," one of his new songs cowritten with Raymond: "I need the ones I love, Lord/ More and more each day"), and his band will stand close to him to make him feel secure. More importantly, four of his eight adult children will be playing backup. Says his banjo-playing daughter Ashley, 24: "He'll look behind him and see people he loves."

Until his tour begins, Campbell seems content to live day by day, his greatest pleasure a game of golf at the Malibu Country Club. "When I play golf, I don't worry about anything," he says. At home Campbell loves looking at old pictures. "All the photos have emotions tied to them," says Kim. "And he likes to be in the same room I'm in. I link him to the present."

No matter what the future holds, right now Campbell just wants the world to know that today, he's "happier than a jackrabbit." Says the legend: "I still love making music. And I still love performing for my fans. I'd like to thank them for sticking with me through thick and thin ... for putting up with me all these years. I'm at peace with the world. This is the happiest time of my life. "