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“Write what you know,” the old novelist’s adage goes. So Adam Carolla co-wrote, co-directed and stars in a movie about a has-been/never-quite-was comic whose glory days were more than a decade ago and whose “partner,” back in the day, went on to bigger and better things.

The TV show that Bruce, his character, starred in was “The Bro Show.” His co-host, played by Jay Mohr, went on to host a late-night TV chat show and still tries to help Bruce with a gig. The real Carolla starred in “The Man Show,” and Jimmy Kimmel, his co-star in that, hosts a popular late-night TV talk show.

Let’s hope the rest of “Road Hard” is fiction, because Bruce is a bitter, comically self-destructive jerk forced to live in the garage apartment of the house his ex (Illeana Douglas) refuses to sell, even as she’s taken up with another man. And that man is played by David Koechner, cast, as ever, as a pudgy, bald and delusional loser. Nobody wants to lose their ex to Koechner.

Bruce gets recognized in airports but not by hotel clerks, who feel his wrath. He unloads on his fellow economy-class passengers and on club owners who short him on his “guarantee,” is humiliated by would-be bar pickups and tries to hide his resentment at others’ success. David Alan Grier plays a pal who just shot a pilot. Watch Bruce/Carolla try not to bite his own lip off as he hides the agony you know this news is putting him through.

Because Bruce is back “on the road,” jetting from Omaha, Neb., to Winnipeg, Manitoba, with stops in Tulsa, Okla., and, if he’s lucky, Boston. It’s where every comic wants to be — in his or her twenties. Bruce, pushing 50, wants to stay at home, hang with his adopted daughter, get some rest.

The funny thing is that Bruce is actually funny, as quick as he ever was. A visit to cowboy country leads to a riff about rodeo bull-riding from the cow’s point-of-view, minding his business when “a 140-pound racist from Wyoming” drops onto his back.

A big fee from a hotel for smoking in his room generates a riotous “Would you rather” rant comparing seedy hotel guests.

The trouble with Bruce is, he’s unfiltered. He’s politically incorrect.

“Black people, the fastest on the planet … the slowest pedestrians.”

He’s never played the game well, and worse, he’s never put the extra work in that it takes to get rich and famous. He’s lazy. A flash of easy success made him that way.

“If I could just scrub my mind of the ’90s.”

Bruce’s plight is not as interesting as his work onstage or the hilarious people Carolla surrounds himself with on-screen. The underused Grier pushes hot buttons and samples his own live act, and character actor Larry Miller sports one grotesque wig after another as Bruce’s agent. He’s called “Baby Doll” because he calls everyone “Baby Doll.”

Baby Doll’s lectures about Bruce’s lack of effort and inability to be charming fall on deaf ears, but he’s got a point. Baby Doll plays the game. Baby Doll is a success.

A mildly unconventional love story drags “Road Hard” to a most conventional conclusion. But Carolla gets a lot of stuff about his career choice off his chest, sometimes hilariously, in this hits-too-close-to-home comedy. And if you’ve ever heard a comic wax all sentimental about going back on tour after film or TV success, Carolla exposes that for the grueling, disappointing lie that it really is.

“Road Hard” – 2.5 stars

MPAA rating: unrated; with nudity, adult language, alcohol consumption

Running time: 1:37

Opens: Friday at the Patio Theater