All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
If you are ill and fasting weakens you, as you have stated, or that you need to undergo medical surgery during one of the days of Ramadan, then it is permissible for you to break the fast, as Allaah says (interpretation of meaning): {But if anyone is ill or on journey, the prescribed period (should be made up) by days later}[2:184]. Then you have to make up that day if you can and if you are expected to recovers from your disease. However, if you have a chronic disease, you do not have to fast, but you do have to feed one poor person for every day that you missed the fast. On the other hand, if you want to fast, you have to refrain from eating and drinking from Fajr time until sunset in your country and not according to the timing of any other country, and it not permissible for you to fast for 12 hours according to the timing of Makkah if the people in your country fast for 15 hours.
Allaah knows best.