What is the way to make up for the many prayers I missed in a previous period of my life when I was not committed, and will my repentance be accepted, and what should I do to feel at ease?

Repentance is accepted no matter how great the sin, and the one who repents from a sin is like one who has no sin. So, if you regret it and resolve not to return to it, and you maintain your prayers, then think well of your Lord. As for the missed prayers, there is a well-known difference of opinion regarding the obligation to make them up. The fatwa issued is that making them up is obligatory. The questioner should follow whoever they trust in their knowledge and piety. If they follow someone who issues a fatwa that making them up is not obligatory, then they should perform many supererogatory prayers and seek forgiveness. If they adopt what we issue a fatwa for — which is the more cautious approach — then they should ascertain the number of prayers they missed and make them up in a way that does not cause them hardship. The Malikis hold that making up two days' worth of prayers with each day is sufficient, and this making up takes precedence over supererogatory prayers. As for the ruling on one who abandons prayer, the preponderant opinion is that they do not disbelieve in a way that takes them out of the fold of Islam.