Can one do Teshuva on Shabbos/yomtov? Regreting sins and saying ASHOMNU, ect. Or only in ones mind?
There is no problem to repent on Shabbos or Yom Tov. On the contrary it is is commendable to repent daily. It is only forbidden to ask a private request that might cause undue anguish.
O:CH 188:6 Mishnah Berurah 22