What is the ruling on masturbating in this manner, and is repentance accepted in this case?

Masturbation is forbidden, and one must repent to Allah for it by feeling remorse, seeking forgiveness, abstaining from it, and resolving not to return to it. Allah, the Exalted, is the Oft-Forgiving; He loves those who repent and forgives all sins, no matter how great, 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.'"