A fantastic book on this subject is
Trustful Surrender to Divine Providence.
Here's a choice quote:
"SICKNESS AND INFIRMITY
We ought to conform to the will of God in sickness and infirmity and wish for what He sends us, both at the time it comes and for the time it lasts and with all the circumstances attending it, without wishing for one of them to be changed; and at the same time do all that is reasonable in our power to get well again, because God wishes it so. "For my part" says St. Alphonsus, "I call illness the touchstone of the spirit, for it is then that the true virtue of a man is discovered." If we feel ourselves becoming impatient or rebellious we should endeavor to repress such feelings and be deeply ashamed of any attempt at opposition to the just decrees of an all-wise God.
St. Bonaventure relates that St. Francis of Assisi was afflicted by an illness which caused him great pain. One of his followers said to him, "Ask Our Lord to treat you a little more gently, for it seems to me He lays His hand too heavily upon you." Hearing this the saint gave a cry and addressed the man in these words: "If I did not think that what you have just said comes from the simplicity of your heart without any evil intention I would have no more to do with you, because you have been so rash as to find fault with what God does to me." Then, though he was very weak from the length and violence of his illness, he threw himself down from the rough bed he was lying on, at the risk of breaking his bones, and kissing the floor of his cell said "
I thank you, O Lord, for all the sufferings you send me. I beg you to send me a hundred times more if you think it right. I shall rejoice if it pleases you to afflict me without sparing me in any way, for the accomplishment of your holy will is my greatest consolation."
And in fact if, as St. Ephraim observes, a mule-driver knows how much his mule can carry and does not try to kill it by overloading it, and if the potter knows how long theclay should bake to be suitable for use and does not leave it longer in the kiln than is necessary, then it would show very little appreciation of God to venture to think that He who is wisdom itself and loves us with an infinite love would load our backs with too heavy a burden or leave us longer than is necessary in the fire of tribulation. We can be quite sure that the fire will not last longer or be hotter than is necessary to bake our clay to the right point"