How can a Muslim regain their faith and feel the sweetness of prayer after a period of discontinuity, given that they do not remember the number of missed prayers they did not perform or performed without ablution? And must they make up all of these prayers, or should they start anew?

You must initiate sincere repentance, which is abstaining from sin, regretting it, and resolving not to return to it. All prayers that were intentionally omitted or performed without ablution must be made up. If you do not know their number, be cautious until your obligation is fulfilled. As for one who intentionally abandons prayer, some scholars have deemed him a disbeliever. Upon his return to Islam, he is not required to make up the prayers, but he must repent and increase his acts of obedience.