How can one get rid of the habit of masturbation and watching pornography, which recurs despite repentance and remorse, and how can one remain steadfast in their covenant with Allah and abandon this sin?

The summary of the answer is:

One must be diligent in prayers and avoid forbidden relationships. Regret after committing a bad habit is a good thing. One must not add other sins, such as browsing pornographic websites. Among the reasons that lead one to fall into this habit are living in a permissive country and studying in a mixed-gender environment. To avoid this, remember that you commit this act in front of Allah and His angels, and He is more deserving of your shyness than people, as Allah Almighty says: (They hide from people and do not hide from Allah). And remember that you do not know when you will die; would you be pleased to die while committing this act? Occupy your time with what is beneficial, seek righteous company, and hasten to marry if you can, as it legitimately cuts off desire. If you cannot, then you must fast, for it is a shield for you.