Is sanctification an upward-bound journey or more like peeling an onion (by which fault and self are more and more deeply revealed)?
Perhaps being more precise in terms of anthropology might be helpful. Distinguishing between affections, appetites, and the spiritual will of a person is important, for only one of these really apprehends the good as such (the will).
With that in mind, sanctification as a direct relation (an x, y slope) speaks to the transformation of the affections. The inner man which judges the goodness of God is strengthened through salvation. And as we seek God’s will, it becomes more and more not just the innermost nous (mind) and will which approve him, but we begin to love him with all our heart, mind, soul, and strength.
Thus the love of God begins to be lived out in the appetites, affections, acts, etc. This is all very imperfectly, but the self is really more and more included in the new life which is a seed. Thus there is growth or, perhaps better, unification through love. The self is more united to God, to his people, and more one simply in itself (as various powers and acts agree more with God).
On the other hand, the image of peeling back layers of an onion speaks to two facets of sanctification.
First, the wound of sin ever remains in this life. That is, the concupiscence (the inclination to sin) which infects our members remains. As more of our life, our habits, our desires are turned over to God, we discover that the root of this concupiscence does not budge, even despite the fact that a particular sin or habit has been done away with.
Thus there is always something to peel away, and in doing so, we uncover a new expression or consciousness of sin in us (even as we grow). We are at times more face to face with the inclination, if not (though also) the acts of sin.
The second thing the image of peeling reveals shows is the nature of what sustains us and partly fuels the work of sanctification. For as we mature, as we become more and more his little children, we discover greater delight in loving God and seeking his face. We experience greater freedom and know more deeply his loving mercy at each step. For we discover that he loves us to our core despite the fact that we are wounded to the core. In this manner there is a growing faith which relies on his goodness and not our own.
So that what is revealed is not just sin, but also the capacity of a heart (aided and directed by grace) to seek God and rest in him. For as Christ who is our life appears (as the heart is purified) we too appear with him! We discover who we are in God through sanctification. And so, the peeling back is a revelation of self and God.
Without all these aspects of sanctification, our growth in this life would be a growth into pride. Instead, it is growth into the new life where we receive his graces with humility.
The Author and Finisher of our faith accomplishes all things well, and we can rely on him to do so.
Perhaps his greatest tool is love itself. For as we love God more, a proper fear of the Lord increases, teaching us not to trust ourselves, to flee sin, and to cleave to him lest we risk loosing the one we love. All this happens even as we increasingly trust him to hold us fast. But because we are more aware of our frailty, we also increasingly recognize our need for him and the dangers of the self–how we are capable of injuring ourselves and others.
Such fear is a gift because its source is love and trust. It is not a worldly fear of condemnation, but rather the fear of the lover who will not risk the loss of their beloved.
Thus love constrains us and trains us. Though we limp, we learn that we can walk in his commandments because we love him, even when our love and our affections are imperfect. So that even if our hearts condemn us, we know that God is greater than our hearts, and he knows everything (1 John 3:20). Such deepens our repentance, our awe, and our life with God.