I feel for you. Obsessing over anything is the pits. It will eat you up and you can't stop thinking about it.
It does sound like this was a life changing experience for you. You are growing as a person for this to be bothering you so much. And sometimes growth is hard. How long ago did this happen? If it was just a few days ago, don't worry things should lighten up in time, but if it happened years ago, that is more of a problem.
Did you know that Amazing Grace was written by a guy that ran a slave ship? He did those horrible things and then it sounds like by the words of the song wanted to change his ways. The first step to becoming a better person is wanting to become a better person. And you have to forgive yourself for the things you are feeling bad about in your past. Doing things we feel bad about is part of the human condition. We can not hold up ourselves up to this ideal of living a perfect life with no marks on our record, no skeletons in our closet.
We all at times in our lives have to humble ourselves and say, I messed up, but I will try in the future to do better next time.
I know having a picture on file at the police has to be hard, it is like it makes you a criminal. Well, I was a victim of a crime once, and having my fingerprints taken by the plolice was a simmilar type feeling. I did nothing wrong and yet the police have my prints on file. If I ever did anything wrong in the future, it would be easier for them to find me with prints to compare with. I did get over it, cause time passed and other problems came along and replace those thoughts with other thoughts. The mind is like that. It can only focus on one thing at a time, so I do believe, as you live your life, this will get smaller in your mind.
Also, sometimes, people are often helped if they feel like they have paid for their mistakes. Volunteer for a soup kitchen, or offer to weed an elderly person's garden. Doing good and giving in anyway always makes your heart feel good. And it sound like your poor heart is hurting. I am so sorry for your pain.