What is the point of the Liberal Democrats? As the candidate who came sixth in last week's Barnsley Central byelection, it is a subject close to my heart. I frequently addressed the theme during the campaign, though the question was never put directly. From the disastrous result a familiar lesson emerged: when the traditional party of protest becomes a party of government, the game changes. Profoundly.

There was a heavy price to pay in Barnsley. In a very safe Labour seat, where most voters have a visceral dislike of Conservative politics and long memories of the bitter miners' strike of 1984-5, the Lib Dems were characterised as having sold their soul for power by doing a deal with the Tories, and were heavily punished.

"You're just bloody Tories with a yellow badge on", or "You're worse than the bloody Tories", were common complaints. "How could you form a coalition with those evil Conservative bastards?" a typical question on the doorstep. Many voters took one look at the yellow rosette on my lapel only to slam the door in my face without a word.

But the most powerful invective was inevitably reserved for our leader Nick Clegg, who was variously described as a coward, a traitor, a turncoat, and Cameron's poodle – usually prefaced by a range of expletives. "He wants shooting" and "he should be hung from the top of the town hall" were among the less helpful suggestions from some Barnsley shoppers.

For the minority of the town's voters (12.4%) who supported the coalition, the Tories took the credit and the Lib Dems took the flak. It is a result our party must avoid in future.

The reality of forming a government last May was dictated by electoral maths. There was no effective choice but to join forces with the Conservatives. Although not overtly stated the coalition in reality is a government of national economic emergency, where both parties have one primary duty – to restore the nation's financial health and deal with the deficit.

In Barnsley, a town heavily dependent on public sector jobs, reneging on the tuition fees pledge attracted most specific criticism, alongside the effects of the cuts. "How can you say one thing then do another – you're absolute liars. I will never, ever vote Lib Dem again," was a typical response.

But despite the rhetoric, if Labour were in power they would also be cutting public services, only more slowly, prolonging the pain.

Yet the question remains: what is the point of the Lib Dems? In part, the answer lies in moving well beyond being seen as enablers and facilitators of a Tory government. We are different. A yellow rosette must always invoke a different emotional and logical response from a blue one. Above all, the Lib Dems must be positive, and not constantly on the defensive over tuition fees and cuts. As much about style as substance, a more proactive, upbeat approach is essential.

Yes, the Lib Dems must demonstrate their real achievements in government more forcefully and more eloquently. Yes, the Lib Dems must champion AV as a fairer electoral system. And yes, the Lib Dems must be seen to be standing up to the Tories publicly, beyond the door of the cabinet room, where victories and concessions, however valuable and hard-fought, are intangible and invisible to the wider public.

As important as the coming battle for AV is, it remains a process, not a policy. Beyond governing successfully, the heart of the Lib Dem mission in competent government should not just be to preserve our separate identity, but to define it more clearly, more acutely, more progressively. The values of the Lib Dem brand need to be enhanced, andto be readily identifiable.Lib Dem policies that are tangible to the electorate and which chime with their aspirations. On everything from social justice to education, taxation to health, defence to welfare, Lib Dems must dare to be different from the Conservatives.

The process should start now, not weeks before the next general election. Lib Dems need to re-engage with voters and start a wide consultation process on future policy. This is not the time for introverted contemplation or talking only to the party faithful. The electorate need to know what makes the Lib Dems different from the Conservatives. They need to know it unambiguously, and they need to know it soon, before the identity of the two parties becomes any more blurred, with blue being the predominant colour.