Both Labour and the Liberal Democrats and Remain generally have been fighting political battled based on their perceptions of others and what they should think and based on a reactionary hindsight.
To phrase it as 'fighting the last battle' is a good way to see it.
Brexit was always a symptom of underlying political issues. It wasn't the cause of them. They crucially both missed this is part of an international trend. Yes its a culture war but preaching to the converted doesn't expand your congregation. Mistake 1.
You can only fight using the same political strategy once. If its rejected by the public you have to change tact, rather than flogging the same dead horse: see Jeremy Corbyn and revoke as strategies. Mistake 2.
Identifying your own weaknesses and being self aware is also crucial. Massive lack of awareness of how you sound to your opposition is a flaw. Mistake 3.
Material reality - identify the problem. These go hand in hand. It's about seeing what the problem is not what you perceive it to be. If a bunch of people are jumping up and down saying 'this is a problem' listen to it. Don't tell them to fuck off out of the party or not to vote for you. Mistake 4.
These rules don't apply in quite the same way to the right. And that's also a fundamental weakness and problem you need to recognise. What does the left and being liberal actually stand for? Mistake 5
Spotting your tactical moment. Knowing when to push on with an ideological idea and knowing when you are taking too many risks are strategic musts. Both Swinson and Corbyn missed their moments on several occasions. Mistake 6.
Corbyn's vision was very inward looking and I have to say the same about Swinson really.
Johnson chased a completely different section of the electorate. He had to appear different rather than appear 'like he was a continuity of what had gone before'. Leaver criticism of Remainers was sometimes phrased as 'continuity remain' and yes I think this was an important point. Mistake 7.