During times of great stress, such as trauma, are energy is more suseptable to damage and influencement. Sometimes a piece of our core energy (call it spirit, soul, what have you) "runs away" and either attaches to the one we lost (in divorce for example) or goes dormant and "hides" within us until we are able to reaccept it. Other times very controlling and/or possessive people can unconsciously "steal" a piece of this energy. Of course, what can be done unconsciously can be done consciously if you still have a connection to them (all relations make links between two people, even merely friendship). The connection is key. People cannot just walk up to another person and take a chunk out of this sort of energy. The connection allows it because naturally, these sort of links exchange small amounts of energy normally. The more emotionally involved, the more "open" the connection is, allowing more damage. Ever heard of "I let myself care too much and got hurt". The more emotion, the more trust, the more "vulnerable" we are.
When someone else holds your energy, it is basically bound to them, they take it in as their own, as if you "belong" to them. When you subconsciously attach your energy to another, it can be retreived by you, once you come to terms by letting go of them and "reclaiming" yourself. Even psychologists/therapists promote letting go and reclaiming who you are. The same goes for dormant energy. Once you can get past what caused it to go dormant, it comes back on its own. This is just a matter of reaching down deep and working things out. Shamans in this case help you along by fighting the battle against your subconscious fears, regrets, etc with you.
But when someone has actually stolen a piece of you consciously or unconsciously...it becomes complicated. Now not only is there your subconscious to deal with, but someone else's manifestations to battle too. This isn't ritual work, this is journeying. Going into a trance, traveling and entering the manifestations of thought and emotion...the stuff of dreams and nightmares. Someone that is unconnected personally to these manifestations have a better chance dealing with them because they are not their fears and insecurites but someone elses. To do it yourself would mean you would need to have the mental and emotional strength and courage to face your own fears head on, manifested right in front of you. No longer just thoughts. Not very many people can do that and I don't advise it for your own sanity. You could do more harm than good.