In my teens I was an ambitious person with lots going for me and it was all swept away by a disability which only left me one option, to stay in education. I'm now an academic with a good career, but I am always thinking 'what if' and have a growing list of 'fantasy exit strategies' but have no funds to get out. But that may change one day.
Regarding ambitions, plans for the future etc I tend not to have any, but instead take every opportunity which comes along, as you don't know where it will lead you. I also make sure that I have a really full life outside of work so I have something to look forward to at the end of the working day. The way I see it, you can do something which is really absorbing, so you get satisfaction through work, or you can do something which enables you to do the things you want to outside of it.
From what you are saying lady_rider the problem seems to stem from this 'ambition' you had rather than anything else. Can you reflect on what you achieved - other than the substance of the thing itself - and whether there is anything you'd like to take forward from it, things you've learned about yourself, things you found interesting within the achievement of the ambition that might spark an interest? Maybe some odd random things which you ignored at the time but could get you into new ways of thinking about your future? Could voluntary or community work help your focus? Also, thinking of things that you didn't like about it, or the other jobs you've done, is equally revealing.
Good luck anyway.