Welcome to the forums and thanks so much for reaching out - it is very courageous of you!
I'm really sorry to hear about this incident, which has caused you much pain and hurt. It must be super difficult trying to get over a painful event that seems to keep intruding your thoughts. Issues like these are quite complex to deal with - on one hand you want to completely forget about it, but on the other hand, you can't seem to control your minds habit of thinking about it. How often do you think about this event? daily or just randomly in response to some trigger?
Like you said, distractions and keeping yourself preoccupied is a good way to move away from those negative thoughts. However, all distractions are quite temporary so it's not really sustainable unless you maybe address the issue head on. I guess just acknowledging your past, accepting this is what's happened nothing will change it, and trying to improve and move yourself away from that instance might be helpful. Everyone has past mistakes and regrets that we wish to change but can't. Fixating on a past issue can't change the pass unfortunately. However, as cliche as it sounds, the past doesn't define you. You have your entire future ahead of you. Do you have a strong support system around you that you can talk to this about?
If you wanted some really good support and some helpful advice I would recommend the Beyond Blue Support Service, which is available anytime on 1300 22 4636. Alternatively, you can get in touch with them via their Webchat 3pm-12am AEST here: www.beyondblue.org.au/getsupport. The counsellors are super friendly and can better help you navigate your feelings and offer some terrific support, as well as advice and referrals.
In the mean time I found this article which you might find helpful. It lists out some good points onto moving away from past mistakes.
Please keep us updated if you feel up to it!
Wishing you all the best and sending you positive thoughts.