Thinking affects the world - just as we knew it did when we were young children.
We knew then that thoughts come into our mind from outside our heads, and that our own thinking was known-by and affected the world outside our head.
We knew then that our fear could attract the attention of that-which-is-feared; and indeed could conjure-up that-which-is-feared. Thinking about a bad thing could 'make it happen' - so we tried not to do it.
(The child experiences this being-part-of-the-world spontaneously and passively; yet it is this basic understanding of 'participation' which the Romantic Christian aims to recover consciously and by choice.)
Our thinking is a part of the world, and this is the reason why sin is evil; why our 'feelings' can harm the world.
The modern materialist cannot comprehend why what he thinks or feels 'in the privacy of my own mind' could or should be of any concern to anybody else - and that God 'would not be interested' in such trivial matters as his personal attitudes, fears or desires.
But the world Just Is Made so that our thinking is part of it. Thinking is Not confined to our brain - the idea that thinking is a free-spinning cog, detached from 'reality' is an incoherent delusion.
(...As Owen Barfield explained in Saving the Appearances, 1965).
Therefore, when we (for instance) experience a powerful fear of what the demon-controlled global totalitarian establishment might do to us (as spontaneously happened to me this morning, shortly after waking) - we are both committing the sin of despair and also making that bad outcome more likely.
We make the bad outcome more likely by lending the creative power of our imagination to an evil world picture. By yielding-to despair, we take the side of Satan; and lend him material assistance by our spiritual activity.
And this is why such thoughts come to mind. Since our minds are not cut-off from the world, evil thoughts can be put-into them by evil spirits - as was always acknowledged in the past.
But the flip side of this frightening reality is that every 'defeat' of evil thoughts in our mind (by our will) has beneficial external effects - which is why we do not get evil thoughts all of the time: defeat wounds Satan. Every repudiation of sin somewhat improves the world
All good thoughts, aligned with God's will, tends to make good futures more likely.
Thus we ought to recognize fear and despair as sins, and inwardly reject them - make the inward affirmation that we do not want to sin.
This act of will (of free will) is called repentance; and it has the effect of transferring our creative aid from Satan to God (a double benefit!).
By repentance (recognition and rejection of sin) we have thus rejected the world picture being imposed upon us by demonic powers and their servants (by all means They can muster), we have made an act of affiliation to divine creation, and we have redirected our soul's effort towards God.
However, this identification and repudiation of sin, and commitment to Good, needs to be distinguished from its perversion as 'the power of positive thinking': the idea that we can 'get what we want' if we want it hard enough.
Positive thinking is about imposing our personal will upon the world - getting what we want from the world; and this is an extremely different matter from willing our allegiance to God's ongoing work of divine creating; extremely different from taking the side of Good in the spiritual war of these times.
The positive thinker would - like a Romantic Christian - also try to reject fear and despair; but he would do thins because they are negative emotions that make him feel miserable. The positive thinker is not rejecting fear and despair because these are sins, nor because they aid the attainment of a more complete and extreme demonic anti-creation than we already have.
Neither would the positive thinker be making a commitment to God's plan for reality. He is instead trying to impose his own plan on reality - and this existential selfishness plays directly-into Satan's hands, is immediately woven-into the demonic anti-creation that opposes God.
Both positive thinking and repentance/ affiliation are - in a broad sense - concerned with 'getting what we personally want'. To the modern secular mind this means that one is as good (or bad) as the other - merely different forms of self-gratification...
Nonetheless, there is a distinction between types of self-gratification in terms of their assumed, ultimate metaphysical context.
The distinction is between a secular attitude of aiming at what provides gratification in this mortal life - on the metaphysical assumption that this mortal life is believed to be the only life. Or, by contrast, a Christian attitude of regarding this finite mortal life as followed by an eternal post-mortal life; with this our mortal life gaining meaning from its purpose.
The Christian purpose of this mortal life is assumed to be 'educational': we are intended to learn from this life 'lessons' of importance to the eternity which follows.
A Romantic Christian does indeed want to reject fear, despair, resentment and many other unpleasant feelings; and instead to think positively.
And repentance does have a positive power. To recognize and repent a sin is to starve it of the oxygen it would otherwise obtain from being regarded as a valid emotion.
But the aim of thinking positively goes beyond mortal life into Heaven; and the the ultimate 'positive' is seen as our personal, chosen, active participation in God's creation.