Will God accept the repentance of a servant who pledges to God to abandon a sin, then weakens and repeats it, then repents and supplicates to Him to strengthen him against himself and his weakness? And are life's hardships considered a consequence of those sins?

Allah Almighty commands His servants to repent, accepts it, and rejoices in it. Repentance is accepted as long as there is no impediment to its acceptance. Whenever a sin is committed, one must hasten to sincere repentance and seek forgiveness, even if it happens repeatedly. The important thing is not to resolve to return to the sin, for Allah Almighty says: "And those who, when they commit an immorality or wrong themselves [by transgression], remember Allah and seek forgiveness for their sins - and who can forgive sins except Allah? - and [who] do not knowingly persist in what they have done. Those - their reward is forgiveness from their Lord and gardens beneath which rivers flow, wherein they will abide eternally; and excellent is the reward of the [righteous] workers." Repentance that fulfills its conditions (remorse, ceasing the sin, and resolving not to return to it) wipes away the sin, for the Prophet, peace and blessings be upon him, said: "The one who repents from sin is like one who has no sin." A promise is not an oath and does not incur expiation. Fulfilling a promise is obligatory because not fulfilling it is falling into sin. Therefore, one must strive against oneself and pray for protection [from sin].