Are righteous deeds nullified and is the supplication prevented from being answered due to periods of despair and ill-thought of Allah, especially when the supplication of others less obedient is answered?

Allah knows best what is good for His servant and is more merciful to him. He only diverts something from him for his own good. Supplication (du'a) is entirely good; either it is answered, or evil is averted by it, or it is stored up for the Day of Resurrection. The withholding of an answer to a supplication might be due to a defect in the servant's faith or a sin he insists upon. Therefore, he must repent and rectify his relationship with his Lord. Despair and despondency are grave dangers if they settle in the heart, and one must repent from them and recall Allah's blessings and wisdom. One must repent from the thought of suicide and disrespect towards Allah, and sincere repentance erases sins.