Serve Hashem be'simcha
Do not EVER let the Yetzer Hara make you sad or dejected if you stumble. This sadness can be worse than the sin itself. The Yetzer Hara is hoping to get you depressed and take you away from Hashem and from yourself. We must always serve Hashem be'simcha. This is one of the most fundamental teachings of Chassidus.
The Lechevitcher Rebbe (a student of R'Shlomo of Karlin) once went as far as to say that even if a person just killed someone and the knife is still dripping with blood, but he feels can't stand up and pray Mincha (the afternoon service) with all his strength and with all his heart, then he has not yet tasted from the waters of Chassidus!