In Likutei Moharan II:48, Rebbe Nachman writes that every movement that brings a person out of physicality and closer to God is "very precious." If a person progresses even a minute amount, in the higher world he has traversed huge distances.
Since a person's spiritual level is not static, in addition to being able to elevate ourselves, we are in danger of falling. It's important to learn to deal with that possibility and to do everything in our power to assure that if we do fall, we will not exceed the minimal boundaries set by the Torah. At the same time, when we fall, we must never lose hope of elevating ourselves to higher levels, and even surpassing our previous levels. Very often, a decline is preparation for a significant ascent, as Rebbe Nachman and Reb Nosson often remind us in their writings.
If a person failed to break his fall in time and has transgressed clear prohibitions, he must be prepared to deal even with this. In Likutei Moharan II:12, Rebbe Nachman tells us that when one is in the lowest of depths, stuck in the mire, he has a special opportunity to find the highest levels of holiness which are hidden there - but only if he is firmly resolved to search for God.
Not only is despair pointless, it can also lead to falling to further depths. But how can anyone escape despair when he understands the seriousness of these moral transgressions? How can a person not despair when the Zohar (219b) states that repentance does not help when it comes to the sin of Er and Onan?
Rebbe Nachman states (Sichot Haran 71) that the Zohar should not be understood literally. Repentance always helps, and the main thing is not to repeat one's wrong doing.