Is it the beginning of winter right now where you live?
It is far more difficult to fast during winter, especially at the beginning of winter, cause human body is programmed through millions of years to put weight on during autumn and beginning of winter, to have reserves to survive long and called winter.
It is far more easy, and also far more natural to fast during the spring time and during summer, because our bodies are naturally programmed to eat less and to lose weight during those months.
That may be the reason why you have difficulties fasting right now, if it is winter beginning where you are.
Your body is simply telling you that you are doing something unnatural.