I'm going to avoid my ex
Why are you going to do that?
Because seeing him reminds me of the great times we had before our break up.
It's hard to get over relationships, especially when you were so happy in them while they lasted.  As long as you are not being rude to your ex or to others around you, it's okay to take reasonable steps to avoid painful memories.
The first step is to completely avoid anything to do with him.
If that works for you, I wish you the best.