Despondency is certainly unnecessary. On several occasions, I heard from my revered father-in-law, the Rebbe shlita, that just as a person must not err with regard to his own shortcomings, so too, he must know his own positive qualities.
Experience shows that often minimizing one's own self-worth is one of the tricks of the evil inclination to weaken [one's resolve] or cause a disturbance [and prevent one] from carrying out a positive activity, as each person certainly knows.