It was listening to the kids that did it for me. Eighteen years old, first-time voters, idealists to the bone, all of them saying any self-respecting progressive would vote Liberal Democrat. Nick Clegg was the way forward for those interested in radical change. Why? Because he said so himself. You want those poor banged-up asylum-seeker children released? Vote Lib Dem. You don't want higher tuition fees? Vote Lib Dem. Ditch Trident? Vote Lib Dem.

My older daughter was beginning to sound like one of those idealists. Hoping to go off to university, fire in her belly, lecturing me on why this lot would make it fairer for people like her. All the way to the ballot box we weren't sure whether she'd put words into action and actually vote for them. Even the Guardian was at it. Electoral reform, you want a new system, do the only sensible thing, it urged. As if we all woke up in the morning screaming for proportional bloody representation.

I did scream. No, no, no, no, not the Lib Dems, the party that's for turning at every opportunistic fork in the road. They have history. The campaigns for the Hodge Hill and Leicester South byelections of 2004 revealed a party at home with trying to be all things to all people. In Hodge Hill's predominantly Muslim Washwood Heath ward, a leaflet featured a picture of Charles Kennedy and candidate Nicola Davies surrounded by Asian voters. In the version of the leaflet distributed in a predominantly white ward, there wasn't a brown face to be seen. And in Leicester South, the "opposed to the war" candidate billed as Parmjit Singh Gill in largely Muslim areas became "born and bred" Leicester man Parmjit Gill in whiter parts of the constituency. Well, if it gains a few votes …

But this is different, chorused the young idealists. Now it's you who's the cynic. Cleggy is refreshing, honest, untainted. Look at the way he stands alone, rising above the arguments and vitriol in those televised debates. Every inch his own man. Look at what he says about the Tories: "There is a gulf in values between myself and David Cameron. They have no progressive reform agenda at all – only an unbearable sense of entitlement that it's just their time to govern." Hurrah for straight speaking.

And then came the corrosive whiff of power. Cameron? He's not that bad. University fees? Anybody's entitled to change their mind. Welfare state? Well, dismantling education, health and the benefits system is radical in its own way. And did you see how good Big Dave and I looked on the first day when the sun shone as bright as the Con-Dem future? And d'you know I was even allowed to lay a wreath at the Cenotaph.

For some people, of course, the fact that he took his party into government for the first time in decades will make Clegg a hero. But not for the believers. They saw him turn. And while with Cameron we knew what we'd be getting, the Clegg dupes did not. And it's that the betrayal that really hurts them.

The one personal positive to take from all this: as we walked away from the polling station, my daughter said she hadn't fallen for the Lib Dems – she'd remembered the story of the double dealing. And she'd discovered Clegg had apparently been a student Tory.