How can one definitively quit masturbation and repent to Allah for it, and when is repentance accepted, and does this habit affect sexual health, and how can one restore sexual health?

The ideal solution to quitting a bad habit is marriage. If you are unable to marry, then you must practice chastity, in compliance with the words of Allah Almighty: {And let those who find not [the means for] marriage abstain until Allah enriches them from His bounty.} [An-Nur: 33].

You must strive against yourself and repent sincerely from this sin, for a minor sin combined with persistence is no longer minor. And remember that if sins are repeated and accumulated, they result in the rust (al-ran) that covers the heart. The Messenger of Allah, may Allah's prayers and peace be upon him, said: "Indeed, when a servant commits a sin, a black spot is marked on his heart. If he refrains, seeks forgiveness, and repents, his heart becomes polished. But if he returns [to sin], it is increased until it covers his heart, and that is the rust (al-ran) that Allah mentioned: {No, but that which they used to earn has rusted upon their hearts.}"

Repel evil thoughts before they turn into ideas, then desires, then actions and habits that are difficult to get rid of. Lowering the gaze is a crucial factor in protecting one's chastity. Allah Almighty said: {Tell the believing men to reduce [some] of their gaze and guard their private parts. That is purer for them. Indeed, Allah is Acquainted with what they do.} [An-Nur: 30].

Strengthen your faith by resorting to Allah, making abundant supplications, reading the Quran, reflecting on its verses, accompanying the righteous, attending circles of knowledge, and drawing closer to Allah by performing good deeds.