I find the practical and simple words of St. Jane de Chantal incredibly comforting and useful. She wrote this letter to her own brother, who was the Archbishop of Bourges, and was indeed suffering from mental and physical difficulties:
When you are experiencing some physical pain or a sorrowful heart, try to endure it before God, recalling as much as you can that He is watching you at this time of affliction, especially in physical illness when very often the heart is weary and unable to pray. Don’t force yourself to pray, for a simple adherence to God’s will, expressed from time to time, is enough. Moreover, suffering born in the will quietly and patiently is a continual, very powerful prayer before God, regardless of the complaints and anxieties that come from the inferior part of the soul.