Joan Smith: Kate Middleton: once again, the lady vanishes

What on earth do the Windsors do to women behind closed doors? Two weeks before her wedding, Kate Middleton has started wearing clothes no normal woman in her twenties would give a second glance. Imagine the shopping trip: a few old friends, up in London for the day, going through rails of clothes and picking out... the knee-length suit. The boring court shoes. The lady-like clutch. And collapsing on each other's shoulders in helpless giggles.

Middleton has already adopted the uniform of an older generation. She's also losing weight, maybe as a result of the pre-wedding "nerves" she admitted to in one of those stilted conversations the royals have with strangers, just like her fiancé's late mother in 1981.

Diana Spencer was a chubby teenager when she was photographed at the announcement of her engagement to Prince Charles, even if she had already started to dress like a middle-aged matron. By the time she got married, she was almost invisible in a huge dress that could have come from a little girl's dressing-up box.

Years later I went to look at Diana's dress collection when she put it up for sale, and I was struck by how quickly she went from fairy-tale bride to concealing herself in dresses padded and ruched like discreet body armour. They seemed to offer an insight into inner turmoil, which was hardly surprising in view of Diana's long-standing insecurities and the inflexible institution she had married into.

Like her would-have-been mother-in-law, Middleton is supposed to be a breath of fresh air, a modernising influence on the rather stuffy Windsors. But she's on the receiving end of patronising advice that betrays the hollowness of such claims, including exhortations to avoid outshining her new husband; three decades after Diana turned into the family's star attraction, a prince's wife must know her place. She's also been told to start breeding as soon as possible, as though there were even a remote possibility that she might suddenly remark that she and William are not all that keen, actually, on starting a family.

When it comes to royal marriages, the monarchy keeps well away from women with careers – only the Queen is allowed to have one, and it's a job for life – and Middleton follows the script exactly. It's not clear she's ever had a real job, in the sense of something with a career structure. She seems not to mind becoming a woman who gets into newspapers for carrying an umbrella or wearing the same dress twice, suggesting a paucity of ambition that's genuinely mind-boggling.

In the midst of so much royal wedding tripe, I mean hype, it's easy to forget what a bizarre institution the monarchy is. The family itself is dull, unable (in my limited experience) to hold an interesting conversation and unwilling to depart from protocol; I've mentioned before that the Queen once cut me dead when I said "hello" to her and didn't curtsy. The notion that the Windsors would welcome a woman who could converse knowledgeably on anything from the future of nuclear energy to the performance art of Lady Gaga is simply risible.

These days, all sorts of relationships are available to consenting adults, from cohabitation and civil partnerships to the not-very-popular option of marriage. Women who marry into the royal family voluntarily accept archaic and burdensome restrictions on their everyday lives, trading autonomy for reflected glory.

In 15 days Kate Middleton will cease to exist, replaced by someone who isn't even allowed to keep her first name. In such peculiar circumstances, her middle-aged makeover and weight loss start to look like a metaphor: it's a classic example of shrinking royal bride syndrome.

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