Why doesn't the questioner feel remorse and regret for abandoning prayer and committing sins, as penitents do, and why has his condition changed after he used to cry and supplicate frequently in his youth?

Weeping over a sin is not a prerequisite for regret, as regret is the heart's pain because of it. To cure hardness of heart, you must persistently remember Allah and obey Him, supplicate for your heart to be softened, frequently remember death and the Day of Judgment and its terrors, reflect on Allah's constant watch over His servant, associate with people of piety who remind you of Allah, and strive diligently in seeking beneficial knowledge.