Is there an obligatory expiation for the sin of harassing girls, looking at forbidden things, and watching pornography in the past, and could this be related to what Allah has afflicted him with concerning his wife?

The expiation for an old sin is through sincere repentance (Tawbah Nasuh), abundant seeking of forgiveness (Istighfar), and performing good deeds that wipe out sins. If the repentance is sincere, there will be no punishment in this world or the Hereafter. You must pursue means for your wife's treatment, increase in seeking forgiveness and performing good deeds, for piety (Taqwa of Allah) is a cause for easing affairs and rectifying situations, as Allah Almighty says: "And whoever fears Allah - He will make for him from his affair ease."