Does committing masturbation invalidate one's repentance while performing ghusl with the intention of repenting, and how can one repent after that?

If someone commits a sin and repents from other sins, their repentance from those (other) sins is valid, and they are still required to repent from the sin they have not repented from. If a person returns to a sin they had previously repented from, their first repentance is not invalidated according to the majority of scholars, provided it met its conditions. It is obligatory to repent immediately from a sin by abandoning it, regretting it, and resolving not to return to it. Allah accepts sincere repentance no matter how great the sin, as He Almighty said: "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.'" One should also increase in good deeds, seek forgiveness, remember Allah, associate with the righteous, and stay away from the causes of disobedience.