i guess this goes here

Memories are created by practicing remembering them. That's why in school you are given writing and recalling tasks. If you don't practice recalling something, you have less chance of remembering it. If you concentrate well, your memory will reflect that. If you are not good at concentrating, your memory will reflect that. Your brain is like a computer, and you are the one that programs it :)

If you are worried about affecting the memories of someone who 'hopped into your mind', you might like to consider talking to a professional who works with the mind.

What he does is Repress negative memories. Unfortunately, it affected more then him when he did it magickally.