Every morning is the same now: wake up, read the papers, start the internal dialogue about how much longer Labour party membership is bearable. Reading that sentence back, it looks ridiculously melodramatic. No one has died and millions of people exist perfectly happily without membership of any political party. Lucky them.

But for those of us who have spent decades (in my case more than 40 years) as members of Labour, these are not easy times. I grew up in a strictly atheist household, where the party was akin to religion. My parents were products of the trade union-sponsored Ruskin College; my father worked on Tribune in the 1950s. My childhood was punctuated by Labour events: Saturday mornings churning out newsletters on an ancient Gestetner copier, Labour bazaars, election campaigns and ward meetings.

Becoming, at 16, assistant party secretary in the high-Tory constituency of St Marylebone (the secretary was the Dowager Lady Lucan, no less), was a thankless task, politically. But friendships were formed which remain strong today. The sense of common endeavour and solidarity, regardless of political disagreements, is a powerful memory.

I stayed with hope and confidence through the “dark days” of the early 1980s, lived through the foundation of the SDP, worked at the heart of the first Labour government for 18 years, saw the many strengths and some weakness of New Labour at close quarters, and endured disagreements and disappointments over Iraq and aspects of Blair/Brown education policy. But I never seriously contemplated quitting, and still consider the opportunity to serve a Labour prime minister a great privilege.

So why now? The most pressing reason is Brexit. If there is an election in the next 12 months, I won’t be able to vote for a party that supports or facilitates Brexit. Although Jeremy Corbyn’s speeches on the subject are rare, the latest was unequivocal: “Labour Brexit” can be a good thing for the UK. I profoundly disagree. It is against the party rules to vote against official candidates, so it would be dishonest to do so.

But Brexit isn’t the only reason. The shockingly badly handled antisemitism row has exposed yet again the nasty, cultish, sectarian, “with us or against us” streak in Corbyn’s Labour that sits ill with the party’s longstanding, comradely tolerance of difference, which included Corbyn’s numerous Commons rebellions.

I am conflicted at leaving behind stalwart members of my local party, now doomed to endure long wars of attrition

A short tweet on the subject of my membership dilemma early this week elicited the usual “piss off then” response from supporters of the current leadership. Labour MP Ben Bradshaw shot back: “Don’t leave – that is what they want.” But what have we come to, supporting a party that actively wants members to leave and appears happy to deter potential voters? Elections are usually won by winning over people who previously didn’t agree with you, not by displacing those who did.

The turmoil of the past few weeks – much of which could easily have been avoided by a swift admission (or at least in Friday’s Guardian article) that it was a mistake not to adopt the internationally recognised definition of antisemitism – has also exposed something many of us fear. That Corbyn is completely unsuited to being prime minister.

It is not just the people he surrounds himself with, several of whom were not even in the party when he became leader. It’s that he appears to be unable to hear opposing arguments or seek compromise, and is so sanctimoniously sure of himself that he is prepared to countenance possible terminal damage to the party’s fortunes. Why should his leadership of the country be any different?

Finally, to me anyway, he isn’t even very radical. I have searched in vain for any far-reaching ideas that might disrupt our current market-driven, hierarchical school system – my particular area of interest. But there are none. Without drastic change to current education policy Corbyn’s banal rhetoric about fairness and equality is just hot air.

So why am I still here? The truth is, it is a wrench to leave. Many Labour MPs are my friends, and I am conflicted about the thought of leaving behind stalwart members of my local party, who are now doomed to endure long wars of attrition over petty interpretations of arcane rules with people who appear to hate members of their own tribe more than they do the Tories. “I am not going to let them steal my party,” one told me.

Unfortunately, my deepest and most unpleasant suspicion is that they already have, and we won’t be getting it back any time soon. All the main institutions – the party HQ, the national executive committee and the leader’s office – are in the hands of the intolerant, blinkered hard left. This is unprecedented. If the party rules change dramatically, as is predicted, it won’t be like the 1980s; this time the fight might be in vain.

But I still find it hard to visualise phoning Labour membership services and uttering the words, “I want to cancel my membership.” About 1% of me is waiting for a sign that something will change: a climbdown on antisemitism; the powerful Momentum grouping realising the cult of Corbyn is doomed and seeking a leadership election; a more inclusive approach to moderate members; or, above all, a recognition that Labour’s best interests, and those of the most vulnerable in society, are served by opposing Brexit.

Even at this late stage, I know my predicament isn’t unique. After seeing my tweet last week, many people contacted me in private and public to say they had already quit. One old friend and fellow education campaigner observed: “If you are thinking like that, you have left already.”

If your political home is where your heart is, that may well be true.