Almost every human on planet earth reacts to what's happening to them. Every time you feel a deep pain for whatever reason you will react to this pain to run away from it instead of integrating it. alcoholic will drink to forget, some people will find different hobbies or others might even run after the money hoping it might fix their issues.
We all know that one person that keeps breaking up with their significant other and says "This guy/girl was an asshole, I'm much better off without him/her!". Then two month later if not less you see them bragging about their new relationship and how amazing it is... just to see them crying again few months later.
The reason behind this is that instead of understanding, and accepting what's making you feel bad, we seek something else to avoid getting hurt.
Being aware of what's around you, and getting to know the presence (ie the present moment, the "presence" that never leaves us but forgetting to acknowledge it when we get older) will make all your bad memories come back. For example if in the past you had a bad experience and felt like your only way to exist is to be stronger than anyone else, then you will be put in situations where you have to fight against other people.
We are all carrying emotional baggages and everytime we feel emotionaly unstable it means something in the past wasn't integrated for some reason (often it is because we were too young and unable to understand, and also often because we didn't want to face reality).
Being in the present moment is the conclusion of an effective pratice of mindfulness meditation. Being in the present moment means that you are unconditionally accepting everything that's happening right now, and not only the enjoyable part. How can you expect to enjoy life in general and accept things if you can't even accept things that happened to you in the past or might happen in the future.
We make the mistake thinking that we want to get better, but in reality we want to feel better. We humans think it's acceptable and possible to live an enjoyable life where everything is sunshine and rainbows. It's not. Life will be tough at times. It can be depressing. It can make us feel terrible. But it can also make us feel amazing.
So, next time your bad memories come up don't run away from them. Don't do anything. Just relax, and be content with everything. When you learn to accept things, you can free yourself up and move on. When you try to run away from it, your old demons will always come back to haunt you.
I came up with an image in which every human has its own tree. You start with a really small tree when you're young and you can sit at the top and see the amazing view quite easily. The older we get, the taller our tree gets. It requires more time to get on top, to appreciate the view but unlike when we were younger we carry huges baggages transforming the climb in a nightmare. We all want to get on top, but at the same time we don't want to give up on what's happened to us (or might happen) because we think they are defining us and we would loose our identity by doing so. If you let go, and let your baggages on the floor before climbing up, then you will have no problems getting to the top.
Imagine you can experience life, just like when you were younger. Often thinking about the past or future, but most of the time just enjoying things. Yes it's possible. I think being able to be in the present moment is the best gift anyone could give you. After all the past cannot be undone and the future hasn't even happened yet. Only the present exists. So you better enjoy what's in front of you instead of always thinking about that 'next great moment' you wont even enjoy because when you'll be close to death these are the things you will remember.