I would suggest just going to a counselor and talking about it. It's never good to block things even more when you know they are there and you can deal with them. No matter what age you were it wasn't your fault and you deserve to talk about it. You do not need to put it away and move on, you need to talk about it. Find someone you trust or go to a professional and work through this.
The memorys are always distressing and this is very common, you and he were both underage and it was probably more of a curiosity thing, however it has upset you and stayed in your mind, it is now time to put it away and move on,if it would make you feel better you could speak to your MOm and get her advice, it wasnt a good idea for you to be looking after a younger child at your age in the first place. Get some feedback and talk it out with her that may make you feel better . It was a long time ago now each time it comes up, dont block it think say for 2 minutes and put it out of your mind.