Forgiveness is always a possibility as long as the effort put towards making things right is genuine. Of course, as you said, there is also the possibility they may never forgive. Part of it rests on the type of person that they are, and the other part rests on how hard you work at it, how hard you prove that you mean it when you try to make things right. Or at least to make them as right as you can. As for who can have faith in you, someone who has been down a similar road can. Someone who has seen the glimpse of humanity within you and knows that it's still there can.
I do have a better idea than most of what it is like to live in such a manner for longer than a normal person's lifespan. You are very right to be afraid of all the guilt, depression, pain and everything else hitting you all at once. Mine hit me all at once. The only reason I think I did not lose my mind again was because shortly after that, my consciousness was taken from my body and stored on a coin. Insurance that I would never hurt anyone ever again. When my knowledge was needed, I'd appear as a hologram. Not exactly living, but it also made me realize more fully just how far I had let myself fall. It is not easy to carry the guilt and pain, but it is far better than the alternative for me. I do not want to be that person again, but turning myself into a better person is a long and difficult task to complete. I spent one hundred and twenty years as a monster, and I cannot help but wonder if it will take one hundred and twenty more to get myself back where I was before.