Nicola Sturgeon was – and still is – important, talented, personable and, to many, inspirational. She was also extremely lucky and often wrong, sometimes seriously so. There are examples of all these qualities in her newly published memoir, Frankly. Sturgeosceptics should concede at once that it contains much that is fascinating, especially about her relations with her charismatic mentor turned vengeful enemy Alex Salmond. Starry-eyed Sturgies should equally admit she made several deep and lasting errors that have left behind a divided nationalist party and movement.

The book is more open and touching about private issues than most political memoirs, although Sturgeon deploys these qualities selectively. Many of the intimate reflections are about being a woman in politics. Other memoirs by female politicians – including those of Margaret Thatcher, Hillary Clinton and Angela Merkel – leave such subjects alone.

Sturgeon does not. She writes about her relationships, her miscarriage, her sexuality and her menopause. Yet to me, the single most affecting sentence in the book comes near the end, when she describes her resignation in 2023. “In short,” she writes, in words that should be pondered by male and female rulers alike, “I was exhausted.”

After Sturgeon’s years of power, who would not have been? From the moment she joined the Scottish National party as a teenager, Sturgeon built a career at the most demanding and, for nationalists, the headiest time in modern Scottish history. She was a parliamentarian, a minister, deputy leader and then leader of her party, and finally Scotland’s first minister from 2014 to 2023. Roy Jenkins wrote long ago that eight years at the top is as much as anyone should be permitted in the full-on world that is modern politics, and Sturgeon’s career proves him right.

You can quibble, as some people have, about whether being Scotland’s first minister is really as important and demanding as being a US president or a UK prime minister. Clearly it is not. However, after what the self-confessed workaholic Sturgeon had already crammed in, she was surely right to get out two years ago and prepare for the rest of her life. She is still only 55, and she is entitled to use this book as a pivot from the old to the new, whatever that may be.

Just don’t idealise her, that’s all. The most important thing about Sturgeon’s political career is not whether she was relatable, good on television or better than the men. Pretty obviously, she was all three. It is whether she was right to be a nationalist. In my book, she was wrong. From her teenage years, Sturgeon’s overriding political goal has been to break up the United Kingdom. It still is.

Many of her admirers, especially in England, ignore this fact. But it cannot be ignored, in part because everything else that she did in politics was always ultimately subordinate to it, and in part because, even now, her cause may yet win in the years ahead if British parties and governments allow it to happen.

Sturgeon’s great political talent was her ability to ride the waves of change. Talent was part of that. But good luck also played a huge role. The first major example of this came in 2004, when she over-ambitiously threw her hat into the ring to become SNP leader in a contest most observers thought she would lose. Who knows where the story would have ended if she had fought on? Over a private dinner in Linlithgow, however, Salmond told Sturgeon that he was planning to enter the contest, and proposed a pact in which she would run as deputy and eventually succeed him. Labour was already haemorrhaging votes as a result of the Iraq war. In 2007 the duo took the SNP to power at Holyrood.

Sturgeon quickly became the most popular politician in Scotland. In 2011, the SNP won an unprecedented overall majority. Her second big stroke of luck, nevertheless, was the defeat of the 2014 independence referendum. Salmond resigned in the aftermath, leaving Sturgeon as his unchallengeable successor. SNP membership soared. What could have been a hospital pass was instead a coronation.

A few months later, it got even better. At the 2015 general election, the SNP captured all but three of Scotland’s Westminster seats. Then came Brexit, opposed by the majority of voters in Scotland. This presented the nationalists with a perfect platform to claim that the union was denying Scotland its will. While the Tories became obsessed with Brexit, and Labour turned in on itself, Sturgeon was able to pitch Scotland as a nation that marched to a different and more progressive drum, and to follow the pro-European path.

But the SNP did not have things all its own way. True, the party remained dominant. Electorally speaking, Sturgeon could have titled her book Undefeated. True also, Sturgeon managed to keep the prospect of a second referendum simmering without boiling over. But in other respects the Scottish political and economic landscape was getting far tougher for the SNP.

There were rows about Lockerbie, on which Sturgeon’s book is informative, and about Hebridean ferry contracts and overruns and Scotland’s drug problems – topics on which she has next to nothing to say. Sturgeon’s unequivocal pledge in 2016 to eliminate the educational attainment gap between children from rich and poor neighbourhoods in the next decade is skated over too. Her account of the court case against Salmond is nothing if not bitter. Her gender recognition reforms were dogmatic and divisive. Her legacy in many respects is division, and perhaps fiscal unsustainability. The reminder in Wednesday’s Government Expenditure and Revenue Scotland report that Scotland’s public spending deficit has widened even further than in the UK as a whole is a timely counterpoint to Sturgeon’s version of history.

But the greatest gift that the gods ever presented to Sturgeon and the SNP came in 2019: Boris Johnson’s accession to the UK prime ministership. Sturgeon did not have to make an argument against Johnson. She could merely let people observe the contrast between his sloppiness and her precision, and let them judge for themselves. It would have been the same with Liz Truss, if she had lasted.

Sturgeon used every lever to imply that Scotland was handling the pandemic better than the wider UK. To many in Scotland and in England, the contrast marked Sturgeon’s apotheosis. In England, some yearned to have Sturgeon in charge south of the border too, and had done so during the Cameron and May years as well.

Yet this was to fundamentally misread Sturgeon. It was a misreading that she was smart enough to encourage. Hers is an interesting account. But it is not the full story. She was not seeking to be the negation of Johnson or Truss. Her aim was to be the negation of the union. It is on this that she should be judged, by nationalists and anti-nationalists alike. By that yardstick her career has – frankly – been a failure. So far, at least.

• Martin Kettle is a Guardian columnist

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