I think the act of recognising that you were a 'fairly shitty kid' shows a great deal of growth. Think about it for a second - the worse situation is you not realising this and continuing to be shitty right?
You are who you are today as a function of all your prior actions and experiences. You have the presence of mind to recognise that you behaved poorly in the past, the best thing to do from now is to acknowledge your past transgressions and learn from them to make the most of the future.
It's completely normal to experience the feelings you are feeling - if we didn't regret our past actions, we would never grow as human beings.