What should one do who vowed to God to abandon masturbation, then returned to it? Is this considered a breach of a covenant with God, necessitating fear of Hellfire? Is this practice considered adultery?

Masturbation is not adultery that necessitates the prescribed punishment (hadd), but it is a reprehensible sin. Getting rid of it is easy through sincerity with Allah and strengthening resolve. The remorse and desire to stop that you feel are signs of good. You must occupy your free time, avoid solitude, not succumb to intrusive thoughts, seek Allah's help, and frequently supplicate. The one who repents from a sin is like one who has no sin. As for the covenant, a Muslim must be keen on fulfilling their covenants with Allah, especially if it is about abandoning a sin. If your covenant was an oath, then its expiation is the expiation for an oath.