Soul fragmentation is when your soul is split into multiple pieces, causing weakness, confusion and dependency on others for your survival. The primary cause is trauma, particularly emotional trauma that you're unable or unwilling to face. When something happens that is too painful or frightening to process, you instinctively turn away from it, blocking it out of your conscious awareness. But that experience and associated energy doesn't simply disappear - it gets cordoned off into a fragment that continues to exist separately from your main self. This 'fragment' operates semi-autonomously, making its own decisions of which you are unaware. Ultimately though, you are responsible for it, and it can cause all kinds of trouble, the karma for which comes back to you.
In order for your soul to be separated from a part of itself, there must be a dividing boundary within it. In the physical world, boundaries are made of something you cannot physically move through, such as a wall. In spirit, boundaries serve the same purpose, but they have to be made of something spiritual. In this case the boundary is made of fear.
The soul fragment often exists in a lower-world, stuck in something similar to a dream. The fear that divides you from the fragment can be upheld from either or both sides. There is the reason why you rejected the fragment in the first place, some kind of fear or judgement, whether justified or not. Then there is the abandonment felt from the fragmented part: having become semi-autonomous the fragment now has its own opinions and emotions about what happened to it, which need to be addressed.
For the most part, the fragment can be treated as one would treat a lost child. That being essentially what it is.
The fragment remains at the emotional and mental age as you were when it was split from you. Some can be spoken to and reasoned with, but those from early infancy may not respond to anything except love.
A fragment generally has a limited and childish capacity for understanding, compassion, etc. and yet it still needs to survive. As such, some turn vampiric, latching on to you or others around you. They can grow quite powerful and manipulative, sometimes becoming something that you could misidentify as a demon. Some have powers that can affect the physical world. Yet underneath is an emotionally stunted fragment of a soul, something like a little lost child who does not know where he is. Having nowhere else to go, they cannot be defeated, but only healed.