I post this every few years the day after Yom Kippur because it is so important. This explains why we sometimes go from such a spiritual high to a spiritual low.
“When a person wishes to enter into the service of God…he must at all times advance and rise from level to level. Now, every time a person is about to move from one level to the next, each in his own way, he has to encounter the full force of the [evil inclination] all over again. These are the desires and fantasies and wild thoughts, the distractions and obstacles which are ranged against a person at every moment and try to prevent him from entering the gates of sanctity.
This often leads to great confusion in the minds of honest people who have truly worked hard on themselves. Suddenly they find all these old desires, confusions and obstacles rising up against them. They start thinking that they must have fallen from their level because before this they had had rest from all these desires and confusions… But this is no fall! What is happening is that the time has come for them to advance from one level to the next, each in his own way. This is why all the desires and confusions and obstacles – all the ‘crookedness in the heart’ – are stirred up against them with fresh force. Each time this happens you must fortify yourself and refuse to lose heart. Strengthen yourself against these forces and break then anew.” ~ R’ Nachman of Breslov in Lekutey Moharan