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Keith Urban 's close confidantes think his recent decisions aren't just about turning over a new leaf ... 'cause TMZ has learned some close to him and Nicole Kidman think he's in crisis.

Sources in Keith and Nicole’s inner circle tell TMZ they think this is a midlife crisis for the 57-year-old rocker ... and, the recent separation is just the latest in a string of confusing choices. Getty

Our sources say Urban fired his whole band -- with whom he'd been playing for 25 years -- back in January, a few months after he released his album "High."

The album didn't end up cracking the year-end charts here in the U.S. ... though it did make a couple lists in Australia -- and it was far from the best-selling album he's ever had.

Of course, the most shocking moment came when he d

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