What are the best ways to preserve repentance and the means of protecting it from weakness and relapse? And does not seeking absolution for transgressions against others render repentance completely unacceptable or merely deficient in acceptance?

The path to steadfastness in repentance and avoiding relapse is through seeking Allah Almighty's help, constantly reflecting on the dire consequences of sin, associating with the righteous, shunning the causes of disobedience, and striving against one's self to abandon them. If the servant returns to sin, then let him repent.