the-art-of-moi
It may be better not to do this on Shabbos. One is supposed to be happy on Shabbos and if reading mussar seforim will make one sad they shouldnt read them on Shabbos.
Btw I want to tell you something very important about Teshuvah. If you sincerely regret it thats it you are done. Theres no use living in the sadness and regret. Hashem wants you to do Teshuvah which means sincerely regret what you did and then FORGET ABOUT IT AND MOVE ON. If you sincerely believe that Hashem forgives when you do Teshuvah then you would just let go and let yourself move on. We all make mistakes, some make big ones. It doesnt matter what the mistake you must move on and focus on the good and living positively. Dont dwell on your aveiros and live in the past.