When Jeremy Corbyn called me last September and asked me to be his shadow health secretary, my world felt like it had turned upside down.

I had never harboured ambitions to be in the shadow cabinet. I spent my first five years as an MP avoiding the national media because it just added another layer of complexity to an already difficult job. I had never spoken from the dispatch box. And despite having helped run a campaign to protect services at my local hospital, I knew little about the NHS. But when Jeremy phoned and asked me to serve, I said yes.

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I remember leaving the TV studios in Millbank the following morning after three hours of interviews feeling like I had survived a round with a heavyweight boxer. I was anxious: to do the job right, not to let my party down, not to let myself down. But I also felt excited and liberated. This was a chance for my party to have a proper rethink, a chance to bring together the best ideas from the left with those from the centre.

I hadn’t voted for Jeremy, and weeks before I’d sent an email to my local party members saying why I was voting for Andy Burnham. Yet my instinct told me that accepting the brief was the right thing to do. Make it work. Play a part in providing an effective opposition. Ensure that those in Lewisham who had voted for a Labour government knew their MP was doing her bit, prepared to put the differences of a leadership contest behind her.

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I loved the job. Learning about a whole new area of policy; understanding how things in the NHS worked, and thinking about how they could work better; meeting principled, intelligent people – from the NHS, charities, local authorities and thinktanks – from whom I could learn so much; working with the fiercely bright, committed individuals who joined my already brilliant team; standing up in parliament and giving Jeremy Hunt a run for his money.

So when friends and colleagues asked “Are you enjoying it?” why did every response start with: “Well, enjoy is an interesting word...”? I loved being the shadow health secretary. But I hated being part of the shadow cabinet.

Facebook Twitter Pinterest Heidi Alexander at the medical centre in Lewisham, where she is MP. ‘I loved learning about a whole new area of policy.’ Photograph: Alecsandra Raluca Dragoi/The Guardian

Some of it was the rough and tumble of politics at that level. The leaks that happen after you said something at shadow cabinet. The frustrations of collective responsibility. Not being totally free to say what you think on a sugar tax or on levels of funding for health and social care.

But it was more than that. I hated being a member of Jeremy’s shadow cabinet – because it was entirely dysfunctional.

I tried to not let it affect me. I had enough on my plate trying to hold the Tories to account. I had a great team of shadow ministers, and we worked hard in parliament to ensure that the public knew that when it came to the NHS and social care Labour was on their side. But as time went on, I came to realise that it simply wasn’t good enough.

It wasn’t good enough for Corbyn to routinely defer to his shadow chancellor when confronted with a difficult decision

It wasn’t good enough for the leader to routinely defer to his shadow chancellor when confronted with a difficult decision – a shadow chancellor who on three separate occasions undermined my efforts to agree collective positions on health matters. It wasn’t good enough for the leader to say one thing to me, only for his political secretary to phone a day later and say: “He may have said that, but I know what he really thinks.” It wasn’t good enough for the leader to read his position from a typed up script at shadow cabinet meetings discussing the prospect of military action against Isis in Syria or the EU referendum. And it wasn’t good enough that whenever he appeared on TV, his description of a process, or his analysis of a problem, ended in confusion or despair on the party’s position – article 50, counterterrorism, “7.5 out of 10” on Brexit.

I hated being part of something so inept, so unprofessional, so shoddy. There was no effort to build a team. Good people recruited to his office soon left.

This wasn’t what I had gone into politics to do. It was a joke. My friends and family knew what I really thought, and I couldn’t bear the prospect of saying something different on TV.

The morning I resigned was difficult. I woke to the news that Hilary Benn had been sacked in the middle of the night. There was no way I could stay. I tried calling Jeremy at 7am from my bedroom. I went downstairs to type a letter of resignation on my computer. I tried again to call him and his political secretary but got no answer. So I texted him and told him I had emailed a letter of resignation. I posted it on Twitter at 8.20am.

Heidi Alexander (@heidi_mp) It is with a heavy heart that I have this morning resigned from the Shadow Cabinet. pic.twitter.com/amBRk30RtR

I wanted to hide but I knew I couldn’t. So, it ended where it had started 10 months earlier: a TV interview and my world turning upside down.

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I wasn’t part of a plot. I wasn’t part of a coup. I had tried hard to make it work. A leader who had been willing to engage, support and take difficult decisions, and had been able to build a team, might have made it work. But we didn’t have one. And in Jeremy Corbyn, much as it pains me to say it, we never will. I saw firsthand 10 months of dysfunctional opposition and a Labour party letting down the people it was meant to represent.

When members and supporters receive their ballot papers on Monday they must answer the same question I asked myself during that difficult weekend after the referendum: “Who is best placed to lead our party and become the next prime minister?”

When Owen Smith called me in the week after my resignation to say he was standing, and to ask whether I would support him, I said yes. When he asked me to help run his campaign, I said yes again. Having sat around that shadow cabinet table, he’d impressed me from day one: smart, capable, radical, knew his own mind but open to listening, able to persuade and able to build a team.

Parliamentary democracy only works when the leader of the opposition is willing and able to engage with every issue that lands on their desk. It only works when the leader is able to command the confidence of a reasonable number of parliamentary colleagues. The basic responsibilities of an opposition cannot be performed without it. Jeremy Corbyn has proved he can’t get this. I think Owen Smith could.

If we fail to learn the lessons of the last year, the Labour party won’t just be failing its own supporters. We will be failing the country.