What is the supplication one can make to Allah to repent and be accepted, especially if one fears death due to the abundance of their sins and their feeling that their repentance will not be accepted?

Fear of death is a good thing if it motivates obedience and preparation for meeting Allah, and it is blameworthy if it leads to despair and inaction. A Muslim should frequently remember death to turn away from desires and to turn towards Allah and the Hereafter. Among the beneficial supplications in this regard is asking Allah to grant one life as long as life is better for them, and to take their soul when death is better for them, and asking Allah for a good ending.