Someone who, by fasting, would be putting his life in danger is permitted and commanded to eat and/or drink, even on Yom Kippur. This should be determined in consultation with one’s doctor and rabbi. This law can be found in the Talmud in the final chapter of Tractate Yoma[1] in both the mishna and corresponding gemara. The Rambam[2] equates one who loses his life in order to keep a commandment for which he is not required to do so to someone who commits suicide. Since halacha treats real danger the same as certain death, it would also be a sin to endanger oneself because of Yom Kippur.[3] It is important to understand that following halacha requires discipline. Heroism is not defined by the degree of daring but in the resolve to follow the rules accurately, even against one’s inclination. In cases like this, avoiding risks is true heroism.
When possible, it is proper to eat very slowly (approximately one fluid ounce of food and of liquid in nine minutes[4]). This reduces the severity of what, for other people, would be a violation of Yom Kippur. On all other fast days, anyone who is sick, even without any danger, may eat according to his needs.
Keep up the noble approach to your illness. Accepting hardship with love of HaShem is one of the highest spiritual levels a person can reach. It is indeed an honor to answer your question. May a full cure be found for your condition, and may you continue to serve HaShem with love and with the addition of good health.