How can the heart be rectified and restored to its natural state after a relapse and a return to watching forbidden things?

What is mentioned in the question causes concern about falling into something worse. The way out of this is easy by promptly repenting sincerely whenever Satan tempts, not despairing of Allah's mercy, striving against oneself, following a bad deed with a good one, frequently seeking forgiveness, being truthful in turning to Allah, and relying on Him and seeking His help.