Term for thinking you can't do something?

There is a term I've heard (the closest I remember is "willful incompetence") for someone who is unable to do something because they think they can't and thus refuse to even try.

E.g. The term I'm looking for would describe someone who thinks they are bad at math and so any time they are asked a question they even think is a math question they will only answer "I don't know", even to someone trying to explain that it's not even a math question.

IIRC the term doesn't necessarily imply that the person is in fact capable (despite believing otherwise) of the thing, just that they are so unwilling to try that their actual capability, or lack thereof, is rendered irrelevant.

Edit: the idea I'm trying to remember the name of has some similarity to learned helplessness but is less passive: a person this phrase describes would actively argue against someone trying to convince them they are not helpless.

Edit: the term might be applied to the protagonist in the movie "The king's speech", particularly demonstrated in this bit.

Are you thinking of "defeatism"?

an attitude of accepting, expecting, or being resigned to defeat

"Pessimism" and "resignation" are somewhat similar. If those words don't work, then perhaps you can use them as jumping-off points for a thesaurus search.

"Conscious incapacity", "wilful incapacity", "inferiority complex", "conscious incompetence", something along the lines of "infirmity" or "ineptitude"?

A thesaurus would be more helpful than [my] memory alone.

Wondering if there is context like in psychology, self-serving bias would have someone convince themselves of their incompetence very actively if they have depression or something that would encourage their strong bias, which in this case, would be aimed towards themself.

I think defeatism in another answer is on the right track. But rather than being resigned to defeat, the person you describe actively engages in self-defeating thinking.

self-defeating (adj.)

Serving or tending to defeat oneself or itself : COUNTERPRODUCTIVE: such as

Acting to defeat its own purpose m-w

Can'tstipation involves obstructing one's own creative potential, amid difficult or challenging situations, by holding on to, and refusing to "excrete" (get rid of) the word can't.
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Thus, the self-defeating nature of can'tstipation lies in the fact that it prescribes not even trying to do things one regards as difficult or challenging. This is self-defeating because one will not accomplish anything difficult or challenging due to one's own failure to even try.Elliot Cohen; Critical Thinking Unleashed

What is Self-Defeating Thinking?

Self-defeating thoughts are any form of thought whose underlying intention is to 'put-down' yourself or someone else. These are not occasional or random thoughts, but rather those which are continual and repetitive. You can recognize them by the typical forms in which they present themselves. Self-defeating thinking commonly begins with one of the following phrases:
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I can't
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Kathy Scott; Core of Self