These personality parts form in response to childhood, adolescent or indeed any past events, where the subconscious mind 'writes programs' to help us deal with certain future situations.
Personality fragments (or sub-selves) are personalities within your personality that pop up in certain situations. Examples of personality sub-selves are; the playful child, the ashamed child, the protector, the nurturer, the worrier, the panicked child, the daredevil, the leader, the fighter, the lover, the procrastinator and many more.
For example, imagine a child or adolescent. Every time she reveals her opinion to her family she is called stupid and therefore ignored. Over time she ceases to express her opinions, and in fact becomes anxious about doing so. She feels ashamed of speaking her mind and unworthy of having an opinion.
This scenario can create a 'personality fragment' within the subconscious mind representing this hurt child. As she grows into adulthood, the personality fragment remains un-addressed. Now, every time she is presented with a situation resembling the situation of the past, for example, management at work, anyone older than her, or even any kind of group situation - the personality fragment pops up, takes over the self, leaving her feeling like an ashamed little kid again. Anxiety arises and she cannot express her opinion due to feelings of unworthiness, shame and fear.
Because the initial events that lead to the fragment forming were repetitive, highly emotionally charged and significant, she feels like she is reliving the past trauma. She feels small and helpless just like the child of the past, even as far as feeling child-like in those situations. Afterwards, the true self or another personality fragment will then take over.
The key is to identify when these personality fragments are triggered, and what they feel like. You can then identify them and work with them to heal them. Pinpoint when they were formed and under what circumstances. Go back and understand, accept and forgive players in those past circumstances, and try to give the traumas the benefit of your conscious reasoning of today rather than the reasoning of a child or adolescent.
The real you of today can then take the wheel more often, until it is in complete control of your life. The personality fragments can let the 'true you' of today permanently take the wheel because they trust who you are now. The personality fragments are no longer needed for you to cope with life; the true you can take the wheel.
Please watch this video for a much fuller explanation.
Balancing of the Self: Recognizing Your Subselves, Personality Parts or Fragments