@Worriedfunction
as anything but a deeply internal motivation
I wouldn't describe a helpful sort of confidence as "deep", because, in my view, it has to be specialized, shaped by the object to an extent.
It would work similar if not equal to motivations, manifesting your belief on an emotional level that it would make sense to invest more of your resources into some endeavor, stimulating you effectively.
Attachment to an external sense of achievement, simply by being "attachment" already makes confidence "informed" in a way or sets the foundation for it.
How would one arrive at this confidence? Not directly.
Through self-evaluation, for example.
If it is an activity in relation to which confidence needs to be built, you might ascertain your average levels of performance in it. Or how such performance progresses over time.
Or may progress over time if you influence in a way some of its factors. Tap into similar experiences and related confidence levels and convert them to your case.
Afterwards, once you manage to collect enough material, you will necessarily arrive at some rough general estimation of your relevant qualities, or will see the outline of the path to develop satisfactory levels for them.
The emotional component of such generic estimation that transcends individual instances of activity in question is what will constitute your confidence, for better or worse.
Confidence can be low as well, and this may still be useful.
In terms of what is useful, can it ever be synonymous with truth?
Rarely. The utility is shaped by the definition of truth but isn't equal to it, I think, as it also concerns itself with application.
Truth is more like end in itself. As it is the case with isolated application of Thinking in the way Jung described it.
In our case specifically, the reductionist approach would be best viewed as just descent within a multi-leveled structure of "truth".
Indeed, our mind may be composed of excitations within a set of quantum fields, but such a level of discourse would move us no closer to understanding how the mind works.