I am half a year in a new job and I absolutely hate it. The people are awful, the work isn't very interesting and I know deep down (or not so deep down) that I would never want a long term future there.
I have been looking for new jobs, but I work in a concentrated industry, and finding an equivalent job is extremely difficult (not least because I am looking to leave six months after starting the last one) - the job I left is pretty much the one that would suit me best.
I had a meeting with somebody from my old job after a couple of months in the new job last year, and they checked whether my old workplace would take me back, but the response was no (I'm not sure what the reasoning was).
I'm feeling at a bit of a dead end, but it seems harsh to think that this would be a career ending mistake. I'm thinking of trying to get back in touch with my old job, but cringe at the thought of being told no a second time - it may be that the best they can do is give me advice on what to do (although I am not really sure what that would be).
Would be interested to hear if anybody has ever been in a similar situation and how you resolved it. Quitting the new job without anything else lined up is very much a last resort, but I would consider it if nothing else came along.