Breaking up is hard to do. So is cracking up at Forgetting Sarah Marshall. It’s both a shapeless comedy about being dumped and an odd misfire from current comic tastemaker Judd Apatow.

Apatow’s anomaly here isn’t that he always chooses golden-touch projects like The 40-Year-Old Virgin or Knocked Up. He’s certainly lent his name to a fair share of duds this decade: Kicking and Screaming, Fun with Dick and Jane, Talladega Nights: The Ballad of Ricky Bobby, Walk Hard: The Dewey Cox Story and, only last month, Drillbit Taylor.

It’s that Marshall, unlike those other bad vehicles, tries and fails to cover the same rowdy, relatable and reliable territory as prolonged virginity or unplanned pregnancy.

After TV starlet Sarah Marshall (Kristen Bell) breaks off a near six-year romance with composer Peter Bretter (Jason Segel), Peter finds himself depressed, distracted and disengaged. He can’t enjoy rebound sex, think about anyone else or write any more “dark, ominous tones” for Crime Scene, an eye-rolling CSI-style show on which Sarah co-stars with Billy Baldwin.

At the suggestion of his stepbrother (Bill Hader), Peter takes a needed vacation. But, surprise, there’s Sarah, sunning at the same Hawaiian resort with her new rocker boyfriend, Aldous Snow (Russell Brand). Peter gradually charms Rachel (the winning Mila Kunis), a front-desk clerk at the resort. Finding happiness with Rachel, and putting all of Sarah’s misery behind him, isn’t easy.

But as written, it should be, and that’s the movie’s major problem. Segel is a formidable front man in the film. But, as its writer, his Marshall plan goes awry. What Peter learns about Sarah, and himself, about an hour in that should immediately end any hang-ups he has over her.

As Peter, Rachel, Sarah and Aldous constantly collide into one another in the second half, all that’s left is a conclusion as shapeless and jowly as Peter’s physique.

It’s ironic that Segel’s script takes a thinly veiled jab at Bell’s starring role in the horror-film remake Pulse. Her work here offers no convincing evidence of her charm or charisma as a movie star, let alone a woman Peter can’t leave behind. Segel also humanizes and demonizes her at the whim of his story. He’s much more successful with a revelatory role he’s written for Kunis, coming into her comedic own after That ’70s Show and giving the movie a dose of effervescent estrogen.

At least the film’s riotous moments slide in as occasionally as its pacing and focus. Segel (who co-starred in Knocked Up) has an easy likeability and the look of Matthew McConaughey … if McConaughey abandoned his diet, exercise and self-confidence. He’s funny both when baring his pathetic soul and swaying genitalia (as he does twice, with appropriately awkward vulnerability).

Although their episodic presence seems fitting of deleted scenes, Apatow’s usual bit players — including Hader, deserving of his own breakout lead — also provide strong laughs: Jonah Hill (Superbad, Knocked Up) as an odd resort staffer with an odd sexuality to how he stalks Aldous; Jack McBrayer (Talladega Nights) as a newlywed uncomfortable with his wife’s wilder urges; and Paul Rudd (Virgin, Knocked Up) as a hang-loose surfing instructor in perma-bake mode.

Male or female, there’s rarely any lower feeling than post-partner depression. Forgetting Sarah Marshall only remembers that part of the time, and its feeble finale attempts a message about how sham love that seems stable can distract us from direction in life. The only relatable angst here is that this movie is only marginally better than Ben Stiller’s remake of The Heartbreak Kid.