In case there was any doubt, British Prime Minister Theresa May is proving once again that you can’t fit a square peg in a round hole no matter how much you try or insist you can.

That, you see, is what the past three years of drama, melodrama and, all too often, dark comedy known as Brexit have really been about. The simple story is that its supporters have promised impossible things — namely, that the European Union would let Britain opt out of the bloc’s free movement of people while still letting it opt in to the group’s free movement of goods — that they’ve merely tried to pretend are not.

What has changed now, though, is that even this mass delusion is becoming harder to maintain. Why is that? Well, a big part of it has to do with Northern Ireland — and more specifically, its border with its southern neighbor. The important thing to understand here is that not only is the E.U. a customs union, but it’s a single market, meaning goods can move between countries without having to be checked at the border, because they all share the same rules and regulations. Leaving the E.U., then, would be bad for Britain as a whole — it probably would face shortages of food and medicine that would have to pass through long inspection lines at the border — but it would be even worse for Northern Ireland. That’s because its open border with Ireland, which was one of the biggest achievements of the peace process 20 years ago, would need to be closed except at designated checkpoints.

This, from the government’s perspective, leaves three rather unpalatable choices. It can either pull all of Britain out of both the E.U.'s customs union and single market, and let a real border go up between Ireland and Northern Ireland; it can leave only Northern Ireland in the E.U.'s single market, and let a real economic border go up between it and the rest of Britain; or alternatively, it can keep all of Britain in both the E.U.'s customs union and single market even after it has technically “left” the E.U., to give it time to try to negotiate a deal that would allow it to pull out of everything completely without requiring a hard border between Ireland and Northern Ireland. That last option is called the “Irish backstop,” and as you might expect, it’s the kind of can-kicking exercise that the government prefers. Not least because it would allow the government to say that Brexit had happened when it hadn’t in any meaningful sense of the word.

The bigger problem, though, is that it doesn’t seem as though there’s any possible agreement that would take Northern Ireland out of the E.U.'s customs union but leave its border open — that’s the round peg and the square hole — so there’s a strong chance that this would mean Britain would just stay in the customs union itself indefinitely. In which case, what would be the point of all this? Britain may as well just as stay in the E.U., and try to do something productive for a change. It’s enough of a fear that Conservative Brexiteers have teamed up with Labour MPs to vote down May’s deal in two of the three biggest majorities in the history of Parliament.

The point is that populism might be able to win in the short term, but reality always does in the long term. The question, though, is how long that long term is. It can be a while. Britain’s problem is that it’s a bit like a guy going through a midlife crisis who wakes up one morning and decides he wants a divorce (leaving the E.U.), in part because he has vastly overrated his attractiveness (how important its economy is). Which is to say that he’s under the mistaken impression that everyone is going to want to date him (offer it free trade deals) once he’s on his own. Not to mention that he didn’t count on the fact that sharing custody of his kids (the Ireland-Northern Ireland border) means that he actually isn’t free to do whatever he wants, but will still have to coordinate things pretty closely with his soon-to-be-ex wife (the E.U.).

Now, in the movie version of this story, he’d belatedly realize that he was better off when he was married, and beg his wife to take him back right before they finalized their I-Don’ts. In real life, things are a lot messier than that. Both the Conservative and Labour parties have a lot of Brexit supporters among them, so neither one is willing to take a firm stand against it. At the same time, they’re not so in favor of it that they want to do it unless there’s a plan for what comes after. Indeed, Parliament just voted down a proposal to leave the E.U. even without a new deal in place. So they’re stuck, with the only apparent options being May’s plan that would keep them in the E.U.'s custom union until they manage to strike a deal that they can’t, and extending the deadline so they don’t crash out of the E.U.'s customs union before there is an agreement. It’s the difference between saying you’ve left the E.U. when you really haven’t, and saying you’ll leave it when you really won’t.

But even that non-choice might be too hard for them. This is Brexit, after all.