...I think the confidence that is applied to the word 'believer', is a short-hand term for the extension of Faith that we were given, part of the Grace bestowed on us...
...as a child, you may not be able to articulate why you trust your parents to have your welfare in mind, but you have the confidence that they will because you know of no other way to be.
In Faith, we have no other way to be, than confident --in the same way that He guards His Word, He also guards us from straying fatally...

In fact, a doubtful believer is the term that would be more paradoxical...
Even though examples of that exist.

It is because of the very reason of Grace, that the ability to believe, and be confident, in spite of doubts, is the workings of God within you -- not something that you can generate totally within yourself, by yourself. An outside agency -- in this case, God -- is the guaranteer of your Belief, and instills and strengthens it when needed.

The minute you see it from the perspective of either confidence or belief being of a wholly human engineered feeling, then you have neither confidence nor belief yourself, and are missing the most important element of understanding the dynamics of both.

Humility is not equated with humbleness, although being humble may be an result of humility. Humility is honest self-assessment, true-seeing of the self and where you stand; your condition and your essence; in thought, word, and deed.
We are given a Rock and a Foundation. The ramifications of that are that we have Absolutes rather than a conditional reality...which builds confidence because surety is one of the first things you extropolate from, having Solidity as the base of your cosmology.
Confidence is a result of Humility, one of the Gifts of the Spirit...if you don't know where you stand, how can you testify about the One who placed you where you are?