Repentance is hard for us moderns to understand - and in fact repentance is typically grossly misunderstood.
What happens is that repentance - which is primarily between one person and God - gets mixed up with emotions such as regret, and more-or-less well-meaning and perhaps helpful inter-human (or 'social') actions such as apology and restitution.
I would say that:
Repentance is the acknowledgement that I have sinned; which includes understanding that what I have done is indeed a sin; and the asking of God's forgiveness for my sins with a contrite and sorrowful heart.
In the above circumstances, this forgiveness will always be forthcoming.
What repentance is NOT includes the emotion of regret for what I have done, focused on wishing I had not done it; nor is it to apologize to someone for what I have done - nor is it to ask another person to forgive me - nor is repentance about trying to undo what I have done, to set things right or repair the damage, nor is it to striving to make amends for what I have done.
All these may be (and often are) good things to do in and of themselves, but they are not repentance.
Repentance is essentially between myself and God; and God's forgiveness is a washing-clean, to allow a fresh start - but a spiritual fresh start; and not a fresh start in this-worldly matters, because - sin can never be undone.
God cannot ever make it as if there never had been sin.
But what God can and does do - what repentance can and does do - is to heal us from the effects of sin.