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Glen Campbell's diagnosis of Alzheimer's disease last year shocked the iconic entertainer's fans all over the world. And even though the veteran singer is soldiering on with his Goodbye Tour while the illness chips away at his memory, Glen's daughter, Ashley, who travels in his band with him, says she can sense the great difficulty he has at times.

"I think he realizes [he is slipping away], and it really frustrates him," Ashley tells USA Today. "Sometimes you can see a little bit of sadness in his eye, when he realizes, 'What's wrong with me? Why can't I do this as well anymore?' But most of the time he'll laugh it off. He doesn't like to talk about it."

The 76-year-old will wrap up the Goodbye tour next month, which Ashley fears could affect the disease's progression. "He functions the best on stage," she explains. "I think it's where he feels the most comfortable, because everything is the same for him on the stage. He knows where he is, where the band is, where the audience is and what he's supposed to do."

Alzheimer's may be affecting his mind, but the musician's performances have still been first-rate. "His voice is amazing, and his guitar solos are still blowing me away," says Ashley. "Every once in a while he'll go off the charts and just be improvising so well, that I'll actually forget to play, and then I'll forget to go to the next part ... It's all just in his head. He doesn't read music. He can read chord charts, but it's just natural to him. It's amazing."

Ashley continues to remain optimistic about the time she has left with her dad. "He's doing OK," she explains. "Alzheimer's is a terrible disease. It's just awful and it's really taking its toll. But he's really happy and he's still playing great in the shows and he still loves what he's doing."