Every time I start a new job I feel like I'm probably not qualified, but I'm sort of used to it by now and I can just keep telling myself "you'll do fine" regardless of how I really feel like it.
How I tend see it is that whenever I do start a new job I am "not qualified", as I don't know the project, or any of the specifics in it, but I can grow into it. New tech involved? No problem, I'll just have to learn it. New industry I don't know anything about? No problem, I'll ask, Google, and figure it out. New people, new expectations of me? It happens, I'll survive.
This being said, I've been a CTO for several years in a couple of companies now, prior to that I've been in lead and senior roles for a long time, and I am definitely competent to do the things that I do.
I think being honest with yourself and your colleagues is going to help a lot. Don't claim to know technologies or other things you don't know, openly say you will need to learn about things you don't know. I've gotten a job saying "I don't really know any of the technologies you use, but I can't imagine it to be a big deal for me to learn them" in the interview, and I did do very well at it.
I have no formal education in CS, but that really has not been any kind of a hindrance after I landed my first job. People coming straight out of school to work in the field are incredibly unqualified anyway, as schools do a terrible job at teaching practical skills in the field. I've seen people come from schools that seemed to focus on far future practically sci-fi tech theory (quantum cryptography) and useless things like writing assembler code on paper, instead of writing good quality code and using good collaboration tools. Practically every single person I've met who learned programming etc. on their own has ended up being a lot better at it than people who learn it at a school.
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