How do I know that I regret what I have done, and is the feeling of distress and the desire to go back in time to avoid the sin considered remorse? And if I fulfill the conditions of sincere repentance, will I be treated as one who has not sinned, as stated in the Hadith, "The one who repents from sin is like one who has no sin"? And does this mean ceasing to mention the sin, or experiencing joy and remorse simultaneously?

Remorse for sin is a feeling of tightness in the heart and a wish that the sin had not been committed, and it is not a difficult matter. If Allah blesses you with sincere repentance, then rejoice in that and ask Him for success in steadfastness and acceptance of repentance. Be careful not to let joy lead to conceit, and be fearful of being forsaken, and attach your heart to Allah with fear and hope, and have a good opinion of Him. Forgetting sin is not entirely praiseworthy, and the issue of remembering or forgetting sin has been discussed.