If it is bad to repress terrible memories, then what about GREAT memories? Shouldn't it have the opposite effect? Therefore people, if you have any great memories, don't tell anyone about it!

I think it has more to do with accepting what has happened to you and learning from it. If you have a positive experience you would like to repeat you should take the same steps you did before. If you repress bad memories, then you are in danger or repeating mistakes. This probably doesn't follow for bad things that happened to you without you having agency in them, I wouldn't entirely know about those.