How can one overcome the feeling of unworthiness or not perfecting acts of worship and study, and the fear of failure in one's profession and the Hereafter?

What you feel of hopelessness, despair, and frustration are symptoms of weak faith, and its cure lies in strengthening your faith in Allah, connecting with Him, and supplicating to Him at times of answered prayers. Among the traditional supplications are: "O Ever-Living, O Sustainer of all existence, by Your mercy I seek assistance; rectify for me all my affairs and do not entrust me to myself for the blink of an eye, nor to anyone among people." And among them: "There is no god but You; glory be to You. Indeed, I was among the wrongdoers." "O Allah, I seek Your forgiveness for my sin and ask for Your mercy." "O Allah, increase me in knowledge, and do not let my heart deviate after You have guided me, and grant me mercy from Yourself, for indeed You are the Bestower." Then strengthen your self-confidence and do not weaken in the face of adversities, for the Prophet, peace and blessings be upon him, said: "The strong believer is better and more beloved to Allah than the weak believer, and there is good in both. Strive for what benefits you, seek help from Allah, and do not be helpless. If anything befalls you, do not say, 'If only I had done such and such,' but say, 'It is the decree of Allah, and what He wills, He does,' for 'if only' opens the door to the work of Satan."