How can I emerge from the sadness and depression that afflicted me after repenting of sins I never expected to commit?

Your sorrow over sins is a good thing and a sign of remorse, which is a pillar of repentance. Make this sorrow a motivation for you to obey Allah and distance yourself from sins. Know that sincere repentance erases sins, for "the one who repents from sin is like the one who has no sin." Have a good opinion of Allah and praise Him for guiding you to repentance. And whenever you remember your sin, perform an act of obedience that will raise your status with Allah.