Keith Urban & Nicole Kidman's Inner Circle Suspect He's in Midlife Crisis

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.

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 decided to end his 19-year marriage to Kidman ... even though she didn't want to separate.

We broke the story ... Nicole and Keith have been living apart since the beginning of the summer ... with Nicole taking care of the kids and "holding the family together through this difficult time."

Kidman filed for divorce earlier this week ... asking for primary custody of the kids -- who, documents state, will be with her 306 days out of the year.

Members of Keith and Nicole's inner circle previously told us they think Keith has moved on with another woman ... and, rumors began to circulate when a video of a Las Vegas performance from April began to circulate -- capturing Urban pointing and singing to his guitarist Maggie Baugh.