usually as an intransitive verb

https://dictionary.cambridge.org/dictionary/english/wane

Halina Birenbaum: Life as Hope
It seemed like she was vanishing before my eyes. ... the ceaseless beating, verbal abuse, filth, lice, and sickness of ...

fade away

gradually become thin and weak, especially to the point of death.
"without help, those of us who are ill will surely fade away and die"

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But was there something about their relationship, their marriage, that made you uncomfortable?
She was kind of... She was fading away, day by day. He was so mean to her. It was like she didn't want to live anymore. - https://www.stockq.org/moviescript/T/the-whole-truth.php

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For years I have watched the mental torment she has endured from her own twisted and sick mother. It was as if she was fading away. But I also have had the privilege to watch her prevail and be an outstanding woman, mother and wife. - https://iamclarehreschaksdaughter.wordpress.com/2013/04/05/i...

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For months, 14-year-old Issy had been struggling. ‘It was as if she was fading away,’ says Suzanne. ‘She was monosyllabic. She wasn’t eating or sleeping properly, and getting her out of the house in the mornings for school was becoming impossible because she’d say she had a headache or stomach ache.’ - https://www.dailymail.co.uk/home/you/article-9530127/I-daugh...

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Note added at 4 hrs (2023-02-10 14:32:53 GMT)
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https://ludwig.guru/s/at a galloping pace

sink/fail right before/in front of one's eyes
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sink = to fail in health or strength

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Note added at 34 mins (2023-02-10 10:52:51 GMT)
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OR ...before one's very eyes

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Note added at 45 mins (2023-02-10 11:03:21 GMT)
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When I started as a rape crisis advocate at age twenty, I often felt helpless when victims seemed to be sinking right before my eyes.
https://www.jstor.org/stable/10.5749/j.ctt17w8gfr



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Note added at 1 day 22 hrs (2023-02-12 08:53:59 GMT)
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Option 2: wither away before one's eyes

Here's how Tom Wilemon later described the event in The Tennessean: "She was withering away before their eyes. They watched her slog through marathon workouts every weekday morning at the Green Hills YMCA, her emaciated body leaning on the rails of the Stairmaster.
https://www.arkansasonline.com/news/2022/aug/21/angels-among...

Not only was I withering away right before their eyes, but other symptoms started setting in, too — I had no energy, I was always cold, and I started getting dizzy almost any time I stood up.
https://www.healthline.com/nutrition/recovering-from-eating-...

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Note added at 1 day 22 hrs (2023-02-12 09:14:41 GMT)
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Sometimes people who are very ill or very old lose a great deal of weight, their skin loses elasticity, their muscles diminish, and they are said to be withering away. Usually, they die.
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Wither away means to lose all your shape and figure until one is a shadow of the previous person.
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...and it is used figuratively for a person who is very ill and is losing a lot of weight. “He is withering away”
https://www.quora.com/What-does-wither-away-mean