How can one return to righteousness and faith after despairing of everything and accumulating sins, while maintaining prayers and obligations?

You are on a path of immense good, and Allah's mercy encompasses all things. Allah is Forgiving and Appreciative; He accepts little good deeds and forgives many errors. Have a good opinion of Allah and ponder His beautiful attributes, such as forgiveness and mercy, so that despair and frustration may leave you. Strive diligently in obedience, repent from your sins, start with a little and then gradually increase, and associate with righteous people. What is preoccupying your mind is from the stratagems of Satan, so do not let him overpower you. Seek refuge in Allah from him, and strive in supplicating for steadfastness in faith and guidance.