Is repentance valid if the regret is weak due to the heart's accustomedness to sin, and how can one overcome the hardness of the heart and the rust (ran) that prevents complete remorse?

Regret is the heart's anguish over committing a sin and wishing it had not been done. It is the foundation and perfection of repentance. To attain regret and remove hardness of heart, it is recommended to remember Allah, strive in supplication, read the biographies of the righteous predecessors, and keep company with good people.