For the times that we feel unworthy of God’s love and mercy, St. Francis de Sales:

It is a very fine thing to feel ashamed of oneself when one realizes one’s own imperfections and misery, but the feeling must not drag on lest one lose heart. It is necessary to raise the heart to God with a holy confidence, founded not in our strength but in God. We indeed change, but God never does; He always remains equally good and merciful toward us, whether we are weak and imperfect or perfect and strong. I always say that our misery is the throne of God’s mercy, and so we must realize that the greater our misery, the greater should be our confidence in Him.

– Spiritual Discourses II; O. VI, p. 22

For our times of trial, he writes:

Considered in themselves, tribulations certainly cannot be loved, but considered in their origin, namely in the Providence of the Divine Will which has brought them about, they are to be loved with an infinite love. Just consider Moses’ staff: laid on the ground, it was a ferocious serpent; in the hands of Moses it was a wonder-working wand. In like manner, tribulations in themselves are terrible, but considered as a manifestation of the will of God they are indications of love and delight. Likewise, love either removes the harshness of the trial or renders it lovable.

– T.L.G. Book 3, Ch. 2; O. V, pp. 112-113