When one reads the spiritual lessons from the Fathers and Mothers of our Church, one realizes that they did not hold to a “one-size-fits-all” mentality when it came to spiritual discipline. Often they set forth the ideal, but acknowledge that some cannot attain the ideal, but instead of despairing, these folk need to embrace what they can do. All-or-nothing thinking is not necessarily the most spiritual way, but sometimes is the result of immature or distorted thinking. St. John of Karpathos writes exactly this referring to fasting and some monks who could not keep the fast strictly due to health problems. Even without fasting St. John tells them they can rid themselves of both demons and passions.
Once certain brethren, who were always ill and could not practice fasting, said to me: How is it possible for us without fasting to rid ourselves of the devil and the passions? To such people we should say: you can destroy and banish what is evil, and the demons that suggest this evil to you, not only by abstaining from food, but by calling with all your heart on God. For it is written: They cried to the Lord in their trouble and He delivered them (Ps. 107:6); and again: Out of the belly of hell I cried and Thou heardest my voice… Thou hast brought up my life from corruption (Jonah 2:2,6). Therefore until iniquity shall pass away that is, as long as sin still troubles me I will cry to God most high (Ps. 57:1-2 LXX), asking Him to bestow on me this great blessing: by His power to destroy within me the provocation to sin, blotting out the fantasies of my impassioned mind and rendering it image-free.
So, if you have not yet received the gift of self-control, know that the Lord is ready to hear you if you entreat Him with prayer and hope. Understanding the Lord’s will, then, do not be discouraged because of your inability to practice asceticism, but strive all the more to be delivered from the enemy through prayer and patient thanksgiving. If thoughts of weakness and distress force you to leave the city of fasting, take refuge in another city (cf. Matt. 10:23) that is, in prayer and thanksgiving. (The Philokalia, p. 314)