How can one atone for the sins of theft, lying, and breaking an oath, committed in the past? And will repentance be accepted, and will Allah forgive repeated sins, including watching pornography and masturbating, given the fear of a bad end and straying from acts of obedience?

The door of repentance is open, so you must perform sincere repentance (tawbatun nasuh) from all sins, especially negligence in prayer. If you repent with a truthful repentance, Allah will accept your repentance. You must stop watching forbidden things and masturbating. What will help you with this is occupying your free time with beneficial deeds, keeping company with righteous people, and adhering to remembrance (dhikr) and supplication (du'a). Mere imaginations that overcome you do not count as sins, but you must guard your thoughts. As for the stolen money, since you do not know its owners, donate it to the poor and needy to clear yourself of responsibility.