Subconcious Blockade(s)

Do a search for something called the Dunning Kruger effect. Most that consider it see the more commonly seen part of it where people think they are the best when they are really not but the, possibly unwarranted, confidence carries them through somewhat. The secondary, and less considered, side of that is people that do know a lot merely see what else there is to learn and consider themselves far worse than they possibly are, something worsened by encountering people from the first category. This can go right up to a point where you think you are a fraud or something and wonder why people would have ever let you loose on such things, something called the impostor effect. It is subtly different to the line of thought that runs "everybody feels like an outsider" but it can feel the same.

Neither might describe you terribly well and what you face might be something closer to a traditional fear of failure, possibly not helped by in the first case you once hearing what the fab shop was charging the client and knowing that your screw up could cost bonuses and whatever else (or that drilling a hole 1mm to the right induces a whole different type of stress that causes it to fail and kill everybody). Now by seeing the computer and all it entails and knowing that you will be the one tasked with fixing anything from a cheap case twisting a motherboard to the point it becomes unstable to the latest java update breaking some subtle aspect of your company's workflow. For what it is worth though you don't have to do that, what you do have to have is a skill that medics call triage and apparently it is the same word in German. Short version is narrow down the problem, try some things you can think of to fix it (and the XKCD tech sheet is accurate despite intending to be humorous https://xkcd.com/627/ ) in the time you are willing to spend (if your main job is day to day IT you don't probably want to pull focus and spend 4 weeks hunting down a race condition or something in a programming language you might have just about be able to get compiled at the start of it) and if that fails you will hopefully know either the person to call in or where to start looking.
It may one day turn out there is some gap in your knowledge in an area that some would consider fundamental, however as you have probably done well by someone for some time prior to this and hopefully did not boast of your prowess in the area that has now failed you then you are not going to be a laughing stock. Maybe it will be that you are out of touch with something when that happens, however by that point you will have hopefully found a niche -- 20 years ago COBOL programmers were relics, 7 years ago they were prying them out of retirement with telephone number sized pay packets and considering it a bargain at twice the price, we are seeing something similar now with K&R C programming today and in general IT there are any number of stories of people knowing what were considered legacy and outdated systems that now have a cushy life thanks to being one of the few that knows enough to keep it running.

None of what I say here will probably sink in right away, hell your own words convey that you already know the irrationality of your situation, and doubtless a lack of false confidence will cost you at some point (false confidence is hard to tell apart from true belief and if the person doing the interview is not a technical person then... this is why salesmen are so despised by technical people) but having a term to look up might help at some level.

There is a fairly effective fix but it is not a good one, you see it a lot in high stress and high risk jobs (medic, finance, police/military and the like, and to a lesser extent in organised crime) and I have seen it a few times in IT as well. This is where some try to harden themselves and make themselves lack any emotional attachment to anything. Does not make for great people at the end of it and some then face serious problems some years down the line, and in the case of IT it sees people want to treat other people like computers which does not work so well. I mention it mainly so you are aware in case you find yourself heading down such a path -- not to long ago you had a thread where you were toying with finance and a line there is "only risk what you can afford to lose", that then becomes someone else's money that they can also afford to lose and this is why investors get a percentage cut or bonuses. This will not be helped by meeting those in accounting that consider IT an expense that will be resolved in 5 years when the kids that grew up with a mobile phone as soon as they gained thumb control enter the workforce and can solve their own problems.

Anyway I am waffling when I probably could have left it at the first two lines.

Even if you don't have confidence, you still manage to get over it and produce your job (or sport) correctly.
I first thought it was like "yips", where unconsciously you do bad. but it's not your action which is bad here, it's your self confidence, not what you're capable of.
thought, maybe yips is what make you lose to get "on the edge" of wining/losing, being in a situation where the stress and unknown let you feel something different than just wining easily.

I'm maybe like you for the self confidence part. I can do my job, but always think others are doing better and that I'm not doing well, that I couldn't do anything else that what I'm currently doing (another job, responsibilities, etc.)
I fear doing mistakes and usually block myself from doing something new, staying in the known water, in my daily routine. Avoiding new situation I "could" (that I think will, but probably won't) have issues with.
I'm shy, and instead of "acting shy", I just "don't act" at all. I avoid going to shops to not talk to people.
It's the same with my work or with what I know. (even here, with hacking/gaming, even if it's easier to interact with everyone else on the forum). I feel afraid and underestimated when I see or interact with users knowing better than me. Always fear I said something wrong and that user will "punish" me, correct me or insult me because I mistaken. I might know more than the majority of people, but always fear reaction from others. it's blocking me from acting. I think that's partly a choice, preventing situation which could place me in a bad position to not feel guilty, not entirely subconscious.

Like Fast said, intelligent people are more aware of all the things they don't know yet and can be a problem than other people don't have. It's the same with knowing yourself, the more you know and analyzed your issues and know where they come from, the harder it is to get them cured. If you didn't know, you go see a shrink and he helps you. If you already know your issues, the shrink will not teach you new things, you already knew all he had to say, it's up to you to help yourself. being sentient of the issue is probably harder to come around it until you decide to.