What is the reason for my condition changing from intense commitment and eagerness for acts of worship to laziness and negligence in religious duties, and what is the way to return to the sweetness of faith I once experienced?

Lethargy in performing acts of obedience is natural. The important thing is to remain obedient to the Prophet, peace and blessings be upon him, and to strive against your lower self, supplicate to Allah, seek His forgiveness, and associate with the righteous. Make the month of Ramadan an opportunity to make up for what you have missed, and frequently remember death.