How can sincere repentance be achieved and recurring relapses overcome?

You must seek refuge [with Allah] from incapacity and laziness, and tread the path of righteousness with diligence and determination. Strive against your soul to abandon all sins, seeking assistance through constant remembrance [dhikr] and supplication [du'a], keeping company with the righteous, contemplating the Last Day and Allah's names and attributes, bringing to mind the immensity of His wrath and punishment, and reading books by scholars on the effects of sins. If a sin occurs, do not despair; rather, repent to Him and persist in repentance no matter how often the sin is repeated, until Allah bestows upon you sincere repentance, after which you do not return to the sin.