How can one deal with the painful memories of past sins after repenting from them?

As long as you have repented from watching movies and from the forbidden act of masturbation, you must remain steadfast in your repentance and be diligent in obeying Allah, and avoid paths that lead to sin. There is no blame upon you for what you see in your sleep, for the pen is lifted from the sleeper. Accusing oneself during repentance and fearing a return to sin is a sign of divine guidance, but this must motivate you to act; otherwise, it would be a deception from Satan.