Even when one falls tremendously to wherever he falls, may the Merciful One save us, it is forbidden for him to give up on himself. Teshuvah is high, even above Torah. Therefore despair does not exist in this world. If a person merits all his sins we be transformed into merits. It is a rule, that from all falls and stumblings that exists in reality, one can easily return to Hashem since there is no boundaries to His greatness. One main thing that everything is dependent on is that a person should not give up from crying out to Hashem, begging and pleading to Him at all times.
(Sichos Haran 3)