A Mere Stringency
Sometimes the yetzer hara deceives you by telling you that you committed a grave sin when there was really no sin at all or [at worst you violated] a mere stringency. His intent is that you should feel depressed as a result thereof, and thus be kept from serving the Creator, blessed be He, because of your depression.
You must understand this trickery, and say to the yetzer hara: “I will not pay attention to the stringency you referred to. You speak falsely, for your intent is but to keep me from His service, may He be blessed. Even if there really was a degree of sin, my Creator will be more gratified if I do not pay attention to the stringency that you pointed out [to me] to make me depressed in His worship.
“In fact, I will serve Him with joy! For it is a basic rule that I do not think the Divine service to be for my own sake but to bring gratification to G-d. Though I ignore the stringency you mentioned, the Creator will not hold it against me, because I do not pay attention to it only so that I will not be kept from His service, blessed be He. For how can I negate His service, even for a moment!”
This is a major principle in the service of the Creator, blessed be He: avoid depression as much as possible.
(Tzava'at Harivash #44)