advice, I definitely have this impostor syndrome, at least to a moderate degree. It's hard to be sure about my younger years, as I often really wasn't too competent yet, but now I'm awesome, and yet still feel it. If anything, I feel it more.
I think there are 4 relevant factors: General self-doubt, the broadness of existence, my generalist approach, and the job market.
The first one would make an obvious contribution, and I've mentioned some examples of it before. Another example was the narrative I had in my head until a few years ago that I was "bad with women", which is madness, as I'm clearly gorgeous (with a split vote on the question of facial hair) and generally fine with women, socially. I'm no Casanova, I don't always spot or act on opportunities, and I don't do well in stereotypical courtship scenarios, like bar/club pick-ups and formal dates. But in more relaxed, unforced situations, I'm fine and clearly not "bad with women", or men, as far as I can tell. And yet, for years I had myself convinced that I was. So I can totally believe that some of my current assumptions, like that I'm "bad at dealing with customers/clients," are similarly flawed.
The second factor is obvious when you think about it, but hard to bear in mind while reading through job ads: There are thousands or millions or gajillions of possible skills to be learned, depending how you divide them up, and nobody can learn them all. So it's easy to find fault in yourself if you focus on what you can't do. I find this is worst if you compare yourself with others too much, and especially people in completely unrelated fields. Having lots of really brilliant friends, as I do, compounds the problem.
This is closely related for me to the third factor, my own vague, wishy-washy generalist's attitude to everything. I don't like specializing, I want to explore everything as widely as I can, and I feel left out when I discover some new field that I know nothing about, which means I probably cover quite a lot of skills, and my general knowledge really is excellent, but it also means that I'm definitely not the best at most of those skills. There are always people better at any given thing than me. I chose to study development studies partly because it's such a broad, multi-disciplinary, generalist thing, and I could get my brain into a hundred new things with it. But it also means that I never get to be the top specialist expert: The economists all know economics better, the environmental scientists know the environment better, the engineers know the technical stuff better, the teachers know education better, and even in political science, my first undergrad major, there are those who embraced it more deeply than I did, especially once dev. studies stole my attention. I know quite a bit about all of those fields, certainly enough to give a decent high school-level introduction to each, and probably enough to be fairly useful at a job dependent on them, but I still never get to feel like I'm truly on top of any one of them.
Compounding this, my career history is similarly all over the place, with experience in retail, admin, education and the manual packing of tens of thousands of promotional pencil and ruler sets. Add to this my "hobby" experience with all the skeptical activism, blogging, daily absorption of dozens of science blogs and podcasts, some serious GMing, starship command, and some model-building and -painting. This all gives me an even wider set of skills (though it's harder to prove it all on my CV, which relates to the fourth big factor), but still none I can claim to have thoroughly mastered, nothing I'd feel comfortable claiming to be an expert at. I was going to suggest that maybe I've mastered GMing, but I don't believe that deep down; I run some decent games, but my NPCs are always a bit weakly expressed, I never know the rules properly (whether or not I plan to ignore them) and the plots I generate myself seldom have solid endings. You see how easy it is to self-deprecate even with the fiddliest split hairs?
Each of these other things comes at a cost of further lost specialisation down the already-general development path. My work in education, for example, is great experience to have, but it's the main thing that's kept me away from development for the last 6 years. I've not kept a hand in my field of study in any way, nor maintained regular contact with those who do, to the point that browsing through development blogs now feels like an alien encounter; I struggle to tell what's over-complicated jargon and buzz words (of which there are always plenty) and what's an important concept I'm not familiar with (and will be laughed at by all the other dev. kids for not getting).
Fourthly, there's the brutality of the job market. Too many people, not enough jobs to go around. And according to my mom and sister, who both have a fair bit of HR experience, most job ads are written to be over-demanding, asking for more things than employers really need, so that they can pick an acceptable candidate from among the pool they expect won't quite meet their stated standards, but will still be good enough for the job. From their perspective, I can totally see how this is a safe way of finding adequately competent candidates, but from my perspective, it freaks me out, being advised (by people I trust) that I have to take this leap of faith and ignore the fact that I don't fit the 'Requirements' section in most job ads.
Worse, I'm also told (and my anecdotal experience kind of fits this) that job ads are for chumps, that employers use them as a last resort, and that the real way to get jobs is to just march into places and demand employment (which is, to me, a maddeningly upside down and haphazard way to organise things), or at least get in through connections and word of mouth (also not the way I like to do things, though possibly a healthier way for a society to operate, in some ways). That all adds to my uncertainty and self-doubt, because I have no objective measurement of whether I'm 'good enough' yet, other than the minority of jobs I do get accepted for. And with hardly any employers bothering with rejection letters (I'd estimate only 1%, in my personal experience), it's very hard to judge what specifically I'm lacking that kept me from being selected. So my imagination is free to feed my self-doubt even more.
(As an aside, this desire for clear estimates of my relative competence level is probably why I like the shiny, shiny badges.)
And if I actually want to make a sustainable independent living, which none of these many things has yet provided (and I'm not getting any younger here), I may have to take on some other job in another field, further diluting my qualifications.
It's no good telling me to accept that this is just the way things work; I'm not intending to change reality just by complaining about it, but I'm not going to suddenly stop thinking about how things work, and thus getting worried by what I see. I analyse things, it's one of my sexier features, and I worry about potential hurdles, perhaps sometimes too much. All of this together combines to (sometimes) make me seriously doubt whether I'm capable of doing anything I hope to do, even if my qualifications and experience suggest I should be capable. It's a pretty unpleasant way to feel, and it seems to strike me worst while actively job-hunting.
Annoyingly, there doesn't seem to be a simple, easy solution to impostor syndrome. Just "pushing through" it seems to be inevitable, since it's a mental obstacle, not a real disability or insanity, but talking about it and trying to recognise how I'm being unfair or unhelpful to myself should also help, right? I'm also curious to know how this affects anyone else, and if it gets to you, how you deal with it.