Will God forgive a young man who used to masturbate while unaware of its prohibition, then repented sincerely after learning about it, and how can he reassure his heart to live a happy life?

Whoever commits a sin then regrets it and repents, Allah will accept his repentance, and it is not permissible for him to despair of Allah's mercy, for Allah Almighty says: "Say, "O My servants who have transgressed against themselves [by sinning], do not despair of the mercy of Allah. Indeed, Allah forgives all sins. Indeed, it is He who is the Forgiving, the Merciful."" It is better for one who has repented not to inform anyone about the sin that Allah has concealed for him, for the Prophet, peace and blessings be upon him, said: "Whoever commits any of these abominable deeds, let him conceal himself with Allah's concealment."