How can one reach sincere repentance after repeatedly watching sexual clips and masturbating, and feeling a heavy heart and pain?

You must sincerely repent to Allah and stay away from the internet if it leads to sins. Strive in supplication for Allah to rid you of bad character.

Turn to Allah by observing His commands and avoiding His prohibitions, increase in good deeds, and keep good company. Be mindful of Allah's watchful eye, avoid being alone with the internet, and create a schedule that fills your time with acts of obedience.

If Satan tempts you after repentance, repent again and resolve not to return to sin, and increase in supplication and seeking forgiveness. The Messenger of Allah, peace and blessings be upon him, said: "Allah, Blessed and Exalted is He, said: 'O son of Adam, as long as you invoke Me and hope in Me, I will forgive you for what you have done, and I do not care. O son of Adam, were your sins to reach the clouds of the sky and then you sought My forgiveness, I would forgive you, and I do not care. O son of Adam, were you to come to Me with an earth full of sins, and then meet Me, not associating anything with Me, I would come to you with an earth full of forgiveness.'"

Ask Allah to bless you with a righteous husband who will protect you from forbidden acts. Conceal what has happened to you from sins; for the Prophet, peace and blessings be upon him, commanded a person to conceal the evil deeds they have committed.