Well, when the memories hit back don't force your mind not to think about them. The more you force your mind the more it'll stick. Just let them be and keep doing your things.
Try to be a bit numb-minded by reading things on the internet. I successfully did erase most of my bad memories back to the grade 7 by doing this. And most things I've learnt there too. Okay that's the side effect.
Meditate. When those memories come while you're meditating just pull your attention back to your object. I suggest for beginners use your breath as your object. I'm a beginner at this myself and when I use another object (the colour blue) I got a headache because I'm not good at visualizing things.
Fill yourself with beautiful memories, start making some new ones. How I forget my hard times in the 7th grade were by moving to another school the next year and getting as much good memories as I could, by making new friends, adapting with the new school, etc. It should be easy for you by now because you have good things around you.
One more thing. Get more sleep. In my 8th grade I used to sleep with 2 x 4 hour session. And I did sleep more than that, maybe for 10 hours a day because I had much time back then. Every time I wake up I become numb-minded and do things slowly, except if I find out that I'm late. Sometimes if it's holiday I return to my sleep after I get up.
