In a new memoir titled Gentle On My Mind: In Sickness and in Health with Glen Campbell, excerpted exclusively in this week's issue of PEOPLE, Kim reveals she struggled in secret as the country star battled drug and alcohol addiction for much of their 34-year marriage.
"When he wasn’t drinking, he was the best guy in the world," Kim tells PEOPLE. “But I feared every time he took a drink. I used to tremble, physically shake all over because it had gotten so bad."
In the memoir, Kim, 61, details one particularly scary occasion which occurred after the birth of their first son, Cal, in 1983. Though Glen had agreed at the time to abide by Kim's mandate of no booze in the house, she had forgotten about a series of collector decanters (which typically hold alcohol like whiskey) she had hidden in their home.
"Glen hadn’t," she writes. "He got hold of the bottles I’d put in the linen closet and drank himself into another crazed rage. This one was bad. This one had him pointing a pistol at me. He leveled the gun at me as if he were lining up a target. I froze. Then, without a word, he turned and walked the other way."
"I was scared to death," she continues. "I felt so stupid for having had a baby with an alcoholic. Now our very lives were at risk."
Kim first became attuned to Glen's problems with alcohol on May 28, 1981, the night they had their very first blind date. They went to dinner and a James Taylor concert in New York City, and though Glen was 22 years her senior and thrice-divorced, Kim — then 22 and working as a Rockette at Radio City Music Hall — admits she was swept off her feet.
"It was pretty much love at first sight for both of us," she says. "Especially for me, I think."
By night’s end, though, the sweetness of the evening had begun to turn sour. Back at his suite at the Waldorf Astoria after the concert, Glen started to drink.
"The evening, begun on a light cloud of charm, was suddenly darkening," Kim writes in the memoir. "I fought the darkness. I didn’t want to see what I was seeing. This man turned me on. I wanted the turn-on to last. I allowed a few more kisses, and yes, I was aroused. But no, I couldn’t deny that he was sloppy drunk."
When Kim got home, Glen called to apologize for his behavior. She forgave him, and the pair remained together from that point on (except for a three-month breakup early in their relationship when Glen got back together with ex Tanya Tucker). Still, the problems persisted.
Kim writes that a rider in Glen's contract stipulated that his dressing room be equipped with a full bar.
"After every show, he hit the bottle and hit it hard," she writes. "He went from sweet and humble Glen to belligerent and self-centered Glen."
By morning, though, he'd forget all he put her through the night prior.
"He’d wake me up with a good morning kiss and return to being the loving man with whom I had fallen in love," she writes, adding that Glen was in "strong denial" about his problems.
The pair married on Oct. 25, 1982, and had a blissful honeymoon at the Grand Canyon where Kim writes she "rediscovered the joy" that had initially drawn her to Glen.
After the pistol incident, and after son Shannon and daughter Ashley made them a family of five, Glen tried to face his demons. He stayed sober for 15 years before relapsing in 2003, when he was arrested on suspicion of DUI and aggravated assault on a police officer following a hit-and-run.
"I was so mad," Kim says now. "I was back to trembling again. I was shaking like a leaf with my cellphone just saying, 'Nobody better go get him. Nobody go get him. He needs to sit in that jail. He needs to wake up and sober up in there and face what he's done.' I was so mad."
Glen made a public apology and spent 10 days in jail, then a month at the Betty Ford Center. Afterward he and Kim moved to Malibu to start anew. As to why she stayed through it all, Kim says, "I loved him. And I believed that he was a good man so I was going to stand by him."
"When he committed to me, we were inseparable," she adds. "I couldn't imagine ever being apart from him."
In Malibu Kim started noticing that Glen was becoming more dependent on her. She’d tell him to get dressed for an engagement, and he’d reply, "Okay, Mommy."
"Other signs: Glen repeated himself as he had never done before," she writes. "He also started forgetting lyrics to songs he’d sung thousands of times. I laughed off these lapses as senior moments."
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As the repetitions and forgetfulness got worse, Kim knew she had no choice but to take him to a doctor for evaluation. In 2011, he was diagnosed with Alzheimer’s.
"It absolutely shocked me and scared me to death," Kim says. "But I tried not to show it because I didn't want to upset Glen."
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Glen's condition progressively got worse, and at one point he even forgot who Kim was. So in 2014, Kim made the decision to move Glen to Abe’s Garden, an Alzheimer’s and Memory Care Center in Nashville.
Then on Aug. 8, 2017, Glen took his last breath at the age of 81.
"The unique thing about Alzheimer's is that you lose them twice," Kim says. "You lose them when they lose their memory and then you lose them physically. So I was grieving for many, many years. As more time goes on, it's getting easier. But there’s an empty spot that no one can ever fill, and it's never going to go away."
Now self-isolating at home in Nashville with Cal, now 37, and Ashley, 33 (son Shannon, 35, is in L.A.), Kim hopes her book can bring some solace to others.
"Whatever battle you're facing, whether it's a disease like alcoholism or drug addiction or Alzheimer's, you can't do it alone," she says. "For us, faith was everything. Faith and a sense of humor."
For more from Kim Campbell and an exclusive excerpt from Gentle On My Mind, pick up the latest issue of PEOPLE, on newsstands Friday.