Humans Store Emotions Physically.
Your body is made of cells, the cells are made of molecules, the molecules are made of atoms, and those are made up of whirling bits of energy; ergo, your body is pure energy. We humans store old memories and emotions in our bodies. That's why the releasing of negativity is a physical, visceral feeling of expelling energy, because energy is what it is, what it all is, what we all are. Ask Einstein.

Examples of What to Release.
Fear, anger, rage, abandonment, rejection, guilt, shame, worry, blame, numbness, hurt, hatred, jealousy and disgust are good places to begin. Your unique soul will know which feelings most need releasing, and in which order to do them. You may need to release some of the deepest ones several times to remove ever deeper layers.

What Does it Feel Like?
Expelling negativity is not a negative experience, it is a relief. For example, when you let go of a deep layer of fear, you do not feel afraid, you feel relieved of fear. Letting go of rage does not mean that you'll be shouting and thrashing about. Rather, your feelings of rage simply pass out of you, and it is a relief. The old negative feelings are fully released and expelled outwards. Releasing negative emotions needn't involve experiencing the pain inherent in any individual emotion. You've already experienced way too much of that. Getting rid of them feels like what it is: a release. It's like taking a gorilla off of your back. It's removing old baggage that does not serve you anymore. It actually feels good. And you will feel progressively better afterwards each time.