What should one do who has been afflicted, repented, then returned to sin multiple times, and fears making a covenant with God and then not fulfilling it; is there any repentance for him?

The door of repentance is always open, and Allah accepts the sincere repentance of a servant, no matter how many times their sin is repeated, in accordance with His (the Almighty's) saying: "And it is He who accepts repentance from His servants and pardons sins," and "Indeed, Allah forgives all sins." One who repents from a sin is like one who has no sin. The servant must mend their ways with sincere repentance and steadfastness in obedience, seeking Allah's help. Caution must be exercised against despairing and losing hope in Allah's mercy, for these are among the major sins. If the broken vow was made verbally, then an expiation of an oath (kaffarat yamin) is required for each breach.