Well, mine was a job rather than a career, but...
Doing O levels, I wanted to be a vet. Stepping up to the appropriate A levels, I realised I just didn't have drive/ability, and flunked them badly. I'd applied to do environmental science at uni, but really only because I thought I ought to do something, and it fitted the subjects. So with 2 E's and no uni place, I went out and got the first job I saw advertised, in Bejam (forerunner to Iceland, for the young ones), while I decided what to do next.
12 years later, still in the same rut - no ambition to move up the retail scale, not real idea what else I wanted to do, dreadful inertia. I'd done an OU degree, but it was so general it didn't point to any particular vocation. And then, one day, out of the blue, I thought "I know, I'd like to go away to Uni and study archaeology". Being my day off, I went into town to the careers office, got the form, looked at the options and applied. York gave me an interview, and then a place, and that was that.
I took to it very well. I met some great people, and loved the work, and got a first. I was told I was PhD material, so I did an MSc, and then after a year out working (in cycling promotion, it was someone I met at York who got me into cycling), got department funding for a doctorate. I fancied a career in academia, teaching and so on.
Two years into it, I was starting to lose motivation. I realised how much of lecturing was chasing funding for projects and I'm not a competitive person, and know I'd hate that bit. And I'd done the data gathering, which I liked, and was trying to come to terms with statistics, which I didn't. Then I had a major traumatic bereavment, and the work just went to pot. Everyone understood and a suspension for a year was suggested, which I took in the hope that the motivation would return when I did. It didn't. I quit the PhD 6 weeks before the final final deadline, a few months ago, knowing it was hopeless.
Luckily, I was able to walk into a job, something I'd been doing for the odd day on a casual basis. I collect recycling 4 days a week, and one day a week I carry on working for Velo Vision magazine which I've done for a couple of years. The money is enough, and the recycling job is physical and outdoors and everyone I work with are great people. I leave work at work, and have no ambition beyond staying solvent and saving up enough to go off one day and explore Europe on my bike for a few months.
So, I sort of lurched from job, to almost career, to job again. Along the way I've been very happy, and very sad, but overall I'm happier now than I was at the beginning....