I've dealt with impostor syndrome all my career, having no diploma and having learned on the job. Now with 20+ years of experience, the diploma doesn't matter anymore, but the feeling is still there.
Not sure this is what you are looking for, but here is what helped me overcome it :
one day, I was thinking about how each job relies on an area of expertise. My job relies on the ability to write good code, maintain complex systems, you know the drill... An HR recruiter job relies on being able to find competent people that are a good fit for the company.
And then, the realization dawned on me : if I'm not a good fit or incompetent, it's the recruiter's or the manager's job to know it. I never lied about who I am or what I can do. If I'm inadequate and I'm still there, it's their fault, not mine. Therefore, since I'm still there, that must mean I'm adequate.
Pretty simplistic, but it helped me tremendously. Now, instead of hesitating before asking for a promotion or a new project, I just go for it. My guts scream that I'm not good enough the whole time. But the people whose job it is to KNOW if I am good enough or not, seem to think I am. So I decided to trust them as much as they trust me. They don't question me on my area of expertise, so I don't question them on theirs.
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