Does celebrity turn women mad? Or must you be mad to pursue fame so hungrily in the first place? This was the question hovering above Lady McCartney's spectacular flame-out on GMTV, BBC News 24, US Today, ABC and any other media outlet with available studio time. Highlights of her performance should be played to classes of teenage girls with dreams of WAGdom, every X Factor or reality soap wannabe.

It's a lesson otherwise unteachable, that even if you are beautiful, have acquired the rich husband whose name assures A-list invites, borrowed jewels and Chanel trinkets, and have a huge divorce swag-bag pending, you could end up weeping and railing over scrapbooks filled with cruel words, and be sent ever more sectionable by a chat-show host's wisecrack. And that when you bare your terror, your suicidal thoughts, your outrage at the sheer it's-not-bloody-fair-ness, the world will turn, stare, then laugh its ass off one more time.

Who would want to be famous today? Wealthy, distinguished in your profession, admired by peers, yes. But fame has never looked an uglier trade-off. You can afford to holiday anywhere in the world, but you can't stroll to the corner shop. Not without either styling yourself to perfection every time you open the door (à la Liz Hurley) or having your scraped-back hair and manky fleece vilified in Heat. I once watched the actor James Nesbitt buy a pineapple in my local Sainsbury's. After he had paid, but was still in earshot, the cashiers had a chortle about how much balder and older he looked than on telly and I suddenly appreciated the small pleasure of purchasing tropical fruit unmolested.

The celeb mags have grown hateful of late - the breezy, teasing tone has flown now that the number of titles has proliferated and they must fight for readers with the potency of their bile. Flick this week through grotesque pictures of Charlie from Big Brother being assaulted in a nightclub. Read how Posh is forced to admit she has acne, that her claims that a paparazzo digitally added spots to a photo were all lies. Mena Suvari is snapped crying in a café, a picture of Demi Moore is headlined “You look so old!”

I recently interviewed Pamela Stephenson, comedian-turned-LA psychologist, who explained that celebrity insanity, depression and substance abuse stem from the gap between a star's public persona - the air-brushed image blown up on billboards, screamed for by fans - and his actual self who can't control his kids, pigs out on pizza, can't always get it up. Celebrities worry that they are not the super-heroic, perfect people of their PR legends; that any moment this will be discovered and success ripped away.

Piers Morgan, in his insightful memoirs, rightly berates stars who whinge about fame, the spoilt ingrates who need reminding of their sublime fortune, perhaps by working a few 12-hour shifts in a call centre. But there are those for whom wealth is no compensation for losing all peace of mind. As Just Jack wrote in their song about celebrity culture, Starz in Their Eyes: “Since you became a VIPerson/It's like your problems have all worsened/ Your paranoia casts aspersions/ On the truths you know.” A study of American celebrities - sports stars, musicians, entertainers - found that they were four times more likely to kill themselves than “civilians” (Liz Hurley's term and their average life expectancy was only 58 years.

At least those with a true gift can be reassured by their talent. The talented will always be forgiven - as Sir Paul McCartney knows - whatever the allegations of wife-hitting. And they have some ulterior purpose, an album to write, a movie to shoot. Classically trained Charlotte Church crunched a spiteful remark from girl-band screecher Cheryl Tweedy with “when you can f***ing sing Ave Maria, then have a go”.

But for those with no talent, whose celebrity is founded upon notoriety or marriage or some long-dead career, their fame is a husk. Only constant publicity can shore up their egos, reassure them that they exist at all. Victoria Beckham, her life one uneasy photo opportunity is a real-life Tinkerbell, kept alive only by the whirr of auto-shutters. Yet for all her couture clothes and great wealth, she looks lonely and puzzled, as if unable to work out why, with every fine detail of a perfect life in place, she still isn't happy.

Diana, Princess of Wales, could have chosen to live quietly after her marital breakdown and remained famous without being a celebrity. But she chose the gowns, galas and showbiz friends: that was her royal revenge. And so by column inches she was judged, and judged herself. Relying upon the vagaries and appetites of newspapers to fuel her self-esteem, led her to paranoia and persecution.

Cherie Blair's latter unhappiness began when she sidelined her real talent for law, and tried to placate the press who mocked her looks, by being a more effortful celebrity: slimmer, more glamorous, better dressed. She swapped a concrete career for a will-o'-the-wisp, an illusion and misery.

And Ulrika Jonsson, now her TV career is all but dead, has nothing to sell but her own life. In her recent documentary Am I a Sex Addict? a psychologist concluded that the answer was unquestionably yes, and advised Ulrika to abstain from a relationship for a year. Yet this week she announced that her fourth child was due by a fourth father. Inevitable, really, when her principal income seems to be selling stories to Hello! magazine in a fast-repeating cycle: her new love, engagement, joy at their new baby, wedding, briskly followed by “Ulrika: how I'll survive alone after our break-up”.

The singer Björk said stardom “feels like a service industry” and pushing out babies to propagate new sub-plots in your personal soap is one kind of celebrity duty. But even posing on the red carpet looks a seedy business. At the Baftas once I happened to enter the building as five Hollywood A-list actresses were strutting before photographers: it was a bitter January night, but still they removed their coats and waggled their goose flesh, the world's most highly paid lap dancers. Premiere parties, as top-class ligger Pearl Lowe concluded in her recent memoir, are empty, unfriendly: it's more fun to pay to see the movie with mates.

Lady McCartney should burn her cuttings books, switch off her TV, move to that gorgeous house she bought by a Slovenian lake, conduct her campaigns in obscurity. Fame without talent is no kind of life.

Times Recommends

• Fragility always gets my sympathy vote
• Double it and you get the real jobless total
• Has al-Megrahi paid his council tax?