In shemona esre, I learned, the bracha of hashivenu corresponds to teshuva for negative commandments while slach lanu corresponds with positive commandments. Why do we hit our chest then, I ask, for slach lanu and not for hashivenu? If the minhag of hitting ones chest means that one has sinned in the heart, it would seem to make more sense to hit for the negative things we have done, out of our wayward hearts desires, than for the positive things we have simply not done, out of negligence, perhaps with no thought given to it at all? Perhaps it is because the positive commandment that we are asking forgiveness for, mostly, is bitul torah -lack of Torah study. Since we are obligated to constantly study, if we spend our time doing other things it shows a lack of constant devotion in the heart. Ve'hegata bo yomam valayla.