A year ago today, I remember being very nervous indeed. Along with hundreds of thousands of people, I held my breath to find out whether or not the impossible had happened, whether Jeremy Corbyn – a role model to so many of the new intake of Labour MPs, and a brilliant, principled politician who looked set to be locked out of power forever – was about to become leader of the Labour party.

That initial leadership election transformed British politics, and we are still coming to terms with the birth of a new mass politics. The Labour party’s membership has grown to well over half a million – making it the biggest left-of-centre party in Europe.

The crowds we drew to rallies last year, and to even bigger ones this year, are now becoming activists in local parties, with some decent results. London and Bristol now have Labour mayors, rolling back years of Tory dominance, while our majorities in by-elections have uniformly increased. In May’s local elections, we overtook the Tories in the share of the vote, coming from seven points behind at the last election.

But if you really want to analyse Corbyn’s achievements, it is important not to fall for the cliche that we have transformed the party but failed at “grown-up” conventional politics. In the past year, the Tories been forced into retreat after retreat. On personal independence payments, tax credits, police cuts and Saudi prison contracts, a sharp, oppositional stance has produced results.

As a member of the most diverse shadow cabinet in history, I witness every day the change that is happening at the heart of British politics. As we hold the government to account over the Panama Papers and tax avoidance by Google and other multinationals, and put forward a genuine economic alternative, we are winning the argument that austerity is a political choice, not an economic necessity. With a million food bank visits a year, a housing crisis, public services crumbling and work increasingly precarious, something has to give.

Even those who oppose Jeremy’s politics must understand their basic moral duty to hold Labour together

For many in politics, that change is not a comfortable one, and that undeniably includes some in our own party. No other Labour leader has been faced with the level of disruption and unfriendly briefing that Corbyn has dealt with – and when so much of the debate is conducted in such irresponsible, apocalyptic terms it is no wonder that our poll ratings are at a low ebb.

But as the result of this year’s leadership contest draws nearer, I have increasing faith that Labour will come together again. Even those who oppose Corbyn’s politics must understand their basic moral duty to hold Labour together.

Actor Julie Hesmondhalgh and MP Rebecca Long-Bailey at a campaign rally with Jeremy Corbyn, August 2015. Photograph: Christopher Thomond/The Guardian

Watching Owen Smith’s appearance on the Andrew Marr Show on Sunday morning, what struck me most was how much consensus there now is inside Labour on bread-and-butter issues – on borrowing to invest, rejecting austerity and creating a more progressive taxation system where the rich pay their share. There will have to be debates about how far that agenda of investment goes, and debates too on issues where there is genuine disagreement, but as a movement we are perfectly capable of resolving our differences democratically.

After the leadership election, we must turn this unity in policy into unity of action. The policies that Corbyn has been articulating in the course of his campaign are not just right, they are popular. Public ownership of the railways is backed not just by a majority of voters, but by a majority of Tory ones. Taking on the “big six” energy companies and renationalising the NHS are overwhelmingly popular, as is a fairer tax system.

With so much of the political class reeling from the vote for Brexit, we are putting forward a concrete plan to address its causes – to create a society where no one and no community is left behind, with a £500bn investment programme designed to create 1m good jobs and 1m good homes, at least half of them council housing. Corbyn’s environment policy, released last week, is the most credible answer ever given to the threat of climate change by the leader of a major political party.

As we mark Jeremy Corbyn’s first year in office, and enter the final weeks of this year’s campaign, we should do so in the spirit of optimism, determination and, above all, unity.