I quit once because the only offer was below the legal minimum wage and included lying in some papers. I did find a paying job soon thereafter (6 weeks maybe).
Some other time they just didn’t prolong my contract. They probably knew what I thought about them, which I didn’t always hide. There were other problems. And possibly someone was making room for other people or just making savings. The stated reasons weren’t particularly credible in the light of what happened later. Also, although secrecy was asked of me, the employer leaked some information. I never found out what it really was. I would have to start suspect people who have acted like friends to have devised it, which I didn’t want to do.
Anyway, I felt so much better for being freed of that place. Not to paint them in a bad light. They paid me. They had paid bigger money to people with less qualifications as I learnt, and they had vainly deluded me with the talk about quick raises while recruiting me (possibly no hard promises but more than enough talking about it) but they were paying well. I was probably ungrateful on several levels in what I thought or even said about them (although they were objectively infringing on a number of stuff). I’ll probably need to reexamine my conscience now that we’re talking. But I was happier to be freed of all the stress working for them and with them entailed, all the conundrums and negative emotions. I started a new job several days before the old one ended, I had almost no income for several months until I changed fields but there was always something. When I changed fields, I started making money. Next year I was making almost the same per annum as in the corporate job but with
so much more free time. A year later, I registered my own business. I have a nice website (that I made myself) and my business cards don’t list a job title. They list the name of business which contains my own name, and my name beneath it. (Nobody needs to know they’re home printed.

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By the way, in my old profession, my applications generally remained unanswered as if I didn’t exist. Too bad. I now have a job that I own and which leaves me with so much free time and the ability to arrange my hours on top of it. I can meet people socially, go to libraries, finish early, take a holiday any time I want, attend academic conferences when I feel like it. If clients treat me badly, they aren’t my boss, I can ask them to stop contacting me (and tell them why) and any specific job I have ethical doubts about I can refuse. For the record, I don’t make tons of money. But I have a good financial return of the time invested in projects and I don’t do any errands or unnecessary procedures or idle time on duty.