Today’s Daf Yomi from the gemorra in Taanis (27) says that the Anshei Maamad shouldn’t fast on Sundays because after eating on Shabbos fasting is too severe.
R’ Moshe Schwerd, a Kew Gardens Hills Daf Yomi Maggid Shiur, brought down the Torah Teminah from his Sefer Tosefes Beracha. He says this is the reason that one who eats on Erev Yom Kippur is credited as if he fasted for two days. After eating, the fast of Yom Kippur becomes that much harder to keep and that is also why Rashi says one should eat a lot on Erev Yom Kippur.
The Torah Teminah goes further and uses this principal to explain the gemorra in Berachot (33B) which says “Makom she-baalei teshuvah omdim, tzaddikim gemurim einam omdim sham”, -“In the place where Baalei Teshuva stand, Perfect Tzaddikim do not stand there.” Since Baalei Teshuva have partaken of that which is forbidden it makes staying away from it that much harder and therefore when they do Teshuva, they stand in a place where Perfect Tzaddikim do not.