How can one quit masturbating while feeling regret and remorse, and is thinness caused by it?

May Allah congratulate you on your repentance from watching evil things. Regarding masturbation, it is forbidden, harmful to health, and weakening to the body. The door of repentance is always open for whoever returns to sin after repenting, and one must repent no matter how many times they commit the sin. The faith-based solution lies in guarding one's private parts except with their spouses, and the tangible solution is knowing its health and psychological harms. Fasting is the prophetic remedy for those who cannot marry, as it is a "Wija'" (a shield) that reduces desire.