Thanks @SantaPawsiscomingtofrown
You asked how I resolved my fears. Well... I'm still resolving them!
I was an academic, and the decision to leave happened slowly, then suddenly. It felt like a marriage where there were admittedly quite a few things to work on, disappointments, things that weren't quite right - a lot of strain but still a sense of shared commitment. But then a rupture.
My disciplinary training gave me some transferable skills, as well as leading me to read the sector in a particular way. So that was self-preserving.
But for my academic identity it felt like a bereavement, or - even more dramatically - chopping off a gangrenous arm.
What partly helped was that I had been through a process of choosing to leave or stay at the postdoc stage, with a job offer in a different sector. I chose to stay because I was pregnant, had just got a permanent post, and could see a positive future. I decided at that point that I should always make it a positive decision to stay, rather than carry on out of fear for the future.
Back then, I had a quick chat with an academic recruiter to find out how academic careers might be diversifying and whether I could ever expect to return if leaving at that point. They said that leaving for a few years could be an innovative thing to do. Getting back into a research-intensive university might be tricky, but there would possibly be routes back into the sector more broadly.
I also talked to a friend who had made the transition at mid-career stage. They said that their new career was rewarding and stimulating. They had gradually had enough of the toxic people getting worse and their good colleagues losing heart. For them, the declining value of the pension also mattered. They were very positive and encouraging about making the leap.
This time round, I had a job coach at the time I was beginning to think about whether I should start exploring options. I had also reconnected with old friends outside academia, and a couple of new friends who had superb advice - they were great sources of wisdom.
Because of them, I realised that transitions are extremely common outside HE. And that it was foolish to think that I had any immunity to precarity, just because I'd won permanent posts and previously done well. The traditional professions are all in upheaval.
I also felt strongly that many academic colleagues and friends had/have too much false hope - and that the careers they were going to end up having would be much poorer than those they had been led to expect. We would not get to do research we cared about. Or even teaching that we cared about, for that matter. That we had much less control over the content of their research and teaching than we realised.
The other thing that helped was making a commitment to myself to carry on the research projects I really care about - which I am.
It was devastating; and I've spent hours railing at the unfairness of the structures, assumptions and individual decisions made by others. And I still have occasional days of extreme sadness and questioning. But the sense of loss is getting less and there are fewer sad days. I'm also doing challenging work with a team of people that I actively wanted to join - meeting them and realising I liked them, would enjoy working with them and learning from them - made a big difference. It's good to go into work and see people looking happy and valued.
tl;dr: take advice, keep your eyes clear and open, know your value.
And if you choose to stay, which is entirely reasonable, make it a positive decision to stay. Make it work for you!