Is it normal for the questioner to feel distress, sadness, and despair—despite her repentance, performing expiation, and Umrah—after committing a sin, and what can she do to overcome these feelings?

You did not mention what you did for us to advise you if expiation is necessary. However, if someone whose knowledge and religiosity are trustworthy has given you a fatwa that repentance suffices for you and no expiation is required, then act upon their fatwa and repent to your Lord with sincere repentance. Know that Allah's forgiveness is greater and His mercy is vaster, no matter what your sin is, so have a good opinion of Him, for He is the Forgiving, the Merciful. Allah Almighty says: "Say, 'O My servants who have transgressed against themselves [by sinning], do not despair of the mercy of Allah. Indeed, Allah forgives all sins. Indeed, it is He who is the Forgiving, the Merciful.'" The one who repents from a sin with sincere repentance regrets what they have committed and resolves not to return to it, and they become as if they had not committed that sin, as the Prophet, peace and blessings be upon him, said: "The one who repents from sin is like one who has no sin." Take it easy, repent to Allah, increase your seeking of forgiveness, have a good opinion of your Lord, strive in His obedience, and increase in good deeds that wipe away sins.