When I was a young child I had a tantrum at a motorway cafe. My parents ordered for everyone, selecting baked beans on toast for me. I stamped my foot and demanded I choose for myself. I then proceeded to read the entire menu, holding up everyone, including an increasingly impatient waitress. Finally, I announced my choice: I would have baked beans on toast.

The memory of that episode returned to me while watching Theresa May give her big Brexit speech at Mansion House today. The speech was praised in some quarters for being serious and, by the standards of her government, pretty detailed. The main takeaway was that the prime minister had finally bidden farewell to “cakeism”, admitting that we couldn’t both leave the single market and have unchanged access to it. “Life is going to be different,” she warned.

But the speech also suggested that Brexit could end up rather like my strop at Little Chef. We would put ourselves and the rest of Europe through a great ordeal, only to end up with an arrangement rather like the one we could have had anyway, all for the sake of feeling in control. Except that, in this case, the end result would be both inferior to, and much more costly than, the dish originally on offer.

She could be a Cicero of rhetoric, an Obama of charisma – she would still be caught between the devil and the DUP

May insisted on Britain’s right to diverge from the EU, but then offered copious examples of areas where we would choose not to, whether by submitting to “reciprocal binding agreements”, applying to rejoin as an associate member various European agencies (on medicine, aviation or chemicals), or vowing not to go below EU standards on industrial goods – often paying hard cash for the privilege. To take the restaurant metaphor upmarket a little, May is the diner determined to go a la carte, only to order dishes that were available on the fixed-price menu for half the cost.

Still, what else could she do? For some, May is increasingly an object of pity, subject to demands to show leadership, to be strong, to “lead or go” – demands that rest on the false assumption that another Tory at the helm would be handling things so much better. May’s sympathisers reply that the problem is not the way she is playing her hand, but the cards she has been dealt.

In this view, it’s the situation itself that is impossible. Pity poor Theresa, forced to tell the rest of Europe – as she did again today – that Britain will leave the single market and customs union yet will allow no hard border in Ireland and no border in the Irish sea. It’s like me insisting that I will be beach-body ready by the summer while maintaining my cast-iron commitment to doing no exercise and eating what I want. Solomon himself could not resolve that one.

The fault, say her defenders, lies not in May’s talents, but in the unforgiving realm of political reality, starting with bald arithmetic. She could be a Cicero of rhetoric, an Obama of charisma and a Thatcher of strength, yet still May would be reliant for a Commons majority on hardcore Tory Brexiteers and Democratic Unionists – caught between the devil and the DUP – along with a band of rebel remainers. Those factions’ interests are irreconcilable, and yet each is armed with a veto.

Facebook Twitter Pinterest DUP leader Arlene Foster and deputy leader Nigel Dodds speak to the media in Westminster. Photograph: Yui Mok/PA

Confronted with those facts, any leader would be as paralysed as May. That Tories quietly recognise this reality is affirmed by her rivals’ unwillingness to move against her. They know that, faced with the same puzzle of insoluble illogic, they’d soon be as helpless as she is.

But the temptation to feel sorry for May should be resisted. For May is not a passive victim in this story. If she’s shackled, it’s in chains that she forged herself. Take the fact that she gave a speech at all. The legal blogger and close Brexit-watcher David Allen Green notes that UK ministers, including May, keep making speeches or writing articles “setting out their vision” while Brussels issues formal, semi-legal documents packed with detail. That will happen again next week, when the EU publishes guidelines for the future trading relationship. The EU’s emphasis on process over publicity gives the EU27 an advantage: their texts become the starting point from which the UK has to diverge or, more often, to which it eventually concedes.

There is nothing inevitable about this. No one forced May to deliver yet another speech, rather than issue a British set of trade guidelines, just as nothing prevented her writing a UK version of the draft withdrawal agreement put out by Brussels on Wednesday. Nothing, that is, except the difficulty of getting her own cabinet to agree on a common position.

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Which brings us to the most recent of three cardinal errors that together built the cage that confines May. She is a prisoner of her party’s competing factions because she lacks a Commons majority. She had one, but threw it away – thanks to a decision that was hers alone.

Not only did she call a needless general election; she also led the poorest campaign in living memory, alarming loyal Tory voters and alienating everyone else. If she’s now at the mercy of Arlene Foster, Jacob Rees-Mogg or Anna Soubry, that is very largely her own doing.

What’s more, the impossibility of her situation is partly about time. She has a matter of months to conclude a free trade deal that most experts estimate is the work of at least five years. But whose fault is that? She didn’t need to trigger article 50 a year ago. It was an absurd act of political machismo. She should have worked out Britain’s detailed and agreed stance first, then started negotiations. To do it the other way around was nothing less than political malpractice.

And her first mistake was perhaps her greatest. On taking power she immediately aligned herself with the hard Brexiteers, implying that remainers were “citizens of nowhere” and setting down the reddest lines on the single market, customs union and the European court of justice. It was with those red lines that she painted herself into the current corner.

So we should not shed too many tears for Theresa May. Her predicament is impossible, but it was she who made it so much worse for herself – and for the country.