The holy Imrei Chaim of Viznitz told a story of a certain tzadik who was saying Nishmas on Shabbos morning. Now Nishmas is a very special tefilla where a person expresses his soul's yearning to praise Hashem. So this tzadik's yetzer hara comes to him and says -"How can you say Nishmas when you did this and this aveirah."
The tzadik answered "You are right! I did this aveira and I will continue doing aveiros - but right now I am saying Nishmas..."
Moral - It's OK to mess up and still be a good Jew afterwards. The yetzer hara wants you to think that you are filled with sin and therefore your mitzvos mean nothing.
