Self-deprecation is an oft used strategy to head criticism off at the pass. On the other hand, too much of it and you basically send the message YOU think you're incompetent.
Man, the fun thing is when you're aware that you are incompetent, but your level of incompetency is still several times more competent than the level of competency possessed by everyone else involved with whatever you're doing.
It is a terrible feeling, to have people looking up(ish) and praising you when you don't actually have any idea what the blue bloody hell you're doing, you're just apparently better at operating in that situation than everyone else.
There's no real gentle way to say, "No, I'm not good at this, you folks just really freaking suck." Especially when their level of capability is actually, like, average or something.
Maybe, just maybe, you're not incompetent, and are instead holding "competence" to too high a standard
Maybe it's because everyone's idea of general competence is loosely based off their own experience of competence-ability. Everyone compares everyone else to themselves. More talented people have higher standards because it's become the norm for them to have such talents.
Actually, people who aren't competent tend to overestimate their competence, and people who are - underestimate it.
I was just gonna bring this up. I read about a study that described the process: you test people's maths skills by making them to math problems, then immediately ask how well they think they did.
Later you get the same people to do the same type of math, but different problems, after further instruction. Then you immediately ask each again how well they think they did.
Turns out, people who were bad at math were also bad at predicting their own ability, self-scoring with a huge error compared to the people who actually did better at the math and predicted their results more closely. At one point the people who were really good actually under-predicted their scores. And as people improved at the math, they improved at their predictive abilities in relation to math.
Which is to say, your ability to tell whether you suck is correlated with how much you suck at that thing. And the outcome is that generally people believe they're great at stuff and only actual testing will reveal the truth.
Although I hypothesize that, knowing the above, you could try to fix your self-assessment using an external reference, such as years spent working in a field or studying a topic and accounting for the years of rust since you stopped doing it. Possibly, assume you're bad at stuff that you haven't been assessed in until you actually get tested.