At times, during a fast, Satan is given permission to cause you great anguish. This is in order to test whether you will stand in opposition and overpower the evil inclination. (If you are wise, you will cry out in supplication to HaShem, blessed is He, to grant you additional strength to overpower him. Certainly, HaShem, blessed is He, will grant you the strength to overcome, as our sages, of blessed memory, stated,¹²⁹ “One who comes to purify himself is granted assistance.”¹³⁰) Later, when you complete your fast, you will no longer feel the great suffering you felt then.

footnotes: ¹²⁹ Talmud Bavli, Yoma 38b ¹³⁰ That is, the good inclination has an advantage over the evil inclination, provided that one comes to purify himself. For as our sages, of blessed memory, stated, “One who comes to defile himself is granted the opening to do so. One who comes to purify himself is granted assistance.” In other words, the evil inclination is only granted the opening, whereas the good inclination is granted assistance. (See Ginat Egoz of Rabbi Yosef Gikatilla, translated as HaShem Is One, Vol. 3, Section entitled “The explanation of the motion of mercy.”)

You should also beg the Creator, blessed is He, for mercy, that He should strengthen your eyes from being damaged due to fasting and weeping.