What is the way to sincere repentance with a feeling of remorse after committing sins, especially major ones, and the fear of dying without it?

The cure for sins is sincere repentance to Allah Almighty, no matter how often the sin is repeated or how great it becomes. When its conditions are met, forgiveness is obtained - Allah willing - for Allah Almighty says: "And He it is who accepts repentance from His servants and pardons sins," and for His saying, peace and blessings be upon him: "The one who repents from sin is like one who has no sin." What you feel of fear of dying in sin and the racing of your heart is, Allah willing, part of the regret that completes the conditions of repentance.