Is returning to watching pornography after sincere repentance considered a grave sin, and how can a person deter himself from this disobedience?

Relapsing into a sin after repenting from it is not considered a major sin if it is due to weakness of will and being overcome by desire, as opposed to persisting in it and taking it lightly, which would then make it a major sin. Among what helps restrain the soul from sins are: sensing Allah's constant watchfulness, contemplating the greatness of the One who prohibited the sin rather than the size of the sin itself, and realizing that one sin leads to other sins and hinders success in acts of obedience. And do not be in a state that you would hate for death to overtake you in.