The American essayist HL Mencken said the proper relationship between journalists and politicians was that of a dog to a lamp-post. A generation of journalists has taken the injunction to heart. In the last Ipsos Mori poll, politicians ranked bottom with just 18% of the public believing they could be "trusted to tell the truth" – marginally worse than journalists and bankers. Oddly, this is only a little worse than 10 years ago, despite the expenses scandal.

My own behaviour has not helped, and I am not proud of that. It is an explanation, but not a defence, to point out that the AA's pollsters say 300,000 people have swapped speeding points and that it seemed like a minor matter back in 2003. I should not have done it. The personal consequences were monstrous for my family and me, including two months in prison.

Most of the saga was played out brutally in public, but there are some hidden eddies that reveal how politicians have become so distrusted. My endgame began when Neville Thurlbeck, the chief reporter of the now defunct News of the World, heard gossip that I was having an affair. Rather than cheapskating on the proposed investigation by hacking my phone, the News of the World put me under extensive surveillance by a retired policeman, a more expensive exercise.

Why was News International prepared to invest so much to tail an opposition Liberal Democrat back in 2009? Maybe it was coincidence, but that summer I was the only frontbencher who, with Nick Clegg's brave backing, called for the Metropolitan police to reopen the voicemail hacking inquiry into Rupert Murdoch's empire.

Given that I was falling in love with someone who was not my wife, you might think that it was an act of folly to court Murdoch's hostility, but the journalist in me rebelled. Publish and be damned. If I was not in parliament to speak out when I saw an abuse, why was I there?

The News of the World sparked the end of my marriage, but another Murdoch title, the Sunday Times, then groomed my ex-wife until she told them about the speeding points. The political editor bought dinners, sent flowers, offered breaks at smart hotels, and reassured her that she would not face any unpleasant consequences (such as prison).

Four successive weeks as the splash in the Sunday Times ensured our joint prosecution. The Crown Prosecution Service loves a celebrity trial. It was the end of my political career, and it locked up my ex-wife too. She was just another "burned contact" for the Murdoch press.

The moral of this story? First, none of this would have been possible without my own mistakes. I am no saint (but nor did I claim to be). Second, politicians now have to live with a 24-hour media, which is more intrusive and hurtful for the people they love, and this is having a corrosive effect on how the public view politicians, and politics itself.

There is a new aggression in the tabloid press. John F Kennedy and David Lloyd George would have been toast. I suspect the only reason John Major was able to keep his affair with Edwina Currie secret is that it seemed so improbable. Politicians are human. A quarter of married men have an affair. I doubt the ratio in the Commons is different, but the consequences can be.

The wider lesson is a liberal leitmotif: it is the duty of politicians to combat concentrations of power wherever they are, private or public, business or state. Time and again, Murdoch has used his media muscle to bulldoze a way for his business interests. In 2010 he wanted to buy all of Sky, and needed Vince Cable's approval. His son James even came to lobby me. The implicit offer was: back us, and we will back you.

Under the last Labour government, Murdoch needed clearance for a newspaper group to get into television. News International backed Labour in 1997, 2001 and 2005. Under Margaret Thatcher, he needed approval to buy the Times and Sunday Times, giving him a dominant position. Murdoch backed the Tories in 1979, 1983, 1987 and 1992.

Each government concession made him more powerful, and more difficult to resist. Such is political momentum – and fear. It was a pattern Murdoch first perfected in Australia, as the excellent early biography by William Shawcross shows.

The truth is, politicians are no more venal or self-serving than people outside politics, and often far more high-minded. (Anyone who wants to make money should go into business. You lose money in politics.) But there is something intrinsic to the process of politics that kindles distrust. Politics is about brokering compromises to resolve conflict. Inevitably, everyone ends up settling for something less than they wanted or even thought was essential and principled: people are disappointed. The people who disappoint them are their politicians. It is not ultimately a recipe for trust or popularity. But how else can societies set their priorities, and make choices? Politics matters.

Ultimately, the new media aggression is not just a problem for those individuals directly affected, it is a problem for us all. Media ownership must be more diverse because it is the lifeblood of public debate. If competition policy is not enough, then we should have statutory limitations or even help for small media outfits (as other countries do). It is not only votes that make a democracy, but voices too.