Why doesn't a servant feel the bitterness of his old sins, especially neglecting prayer and fasting, despite his sincere repentance and commitment to acts of obedience, and is it necessary for him to feel that way?

Praise be to Allah for your repentance from major sins, and know that remorse is repentance. Sense the greatness of Allah, reflect on death and hellfire, and fear your sins while maintaining a good opinion of Allah. The people of spiritual discipline have differed on whether the repentant person should remember their sins or not. Scholars have also differed on making up missed prayers and fasts, but the more cautious approach is to make them up. You may follow the opinion of a scholar whom you trust regarding these issues.