I relate a lot to Rainbow's struggles and am doing some similar work with my t, so I want to add my two cents. . .

Experiencing oneself as having "parts" isn't usually a decision that is made. The client doesn't decide whether or not it would be useful to see themselves as having parts. If parts work is being done in therapy, it is usually because the client already experiences themselves as having separate parts of their personality that feel different from what they consider their "normal self." If this is the case, then doing work such as IFS in therapy can be very beneficial.

Usually, if a person has parts, it is because they have experienced some traumatic stuff from their past that was hard for them to acknowledge, experience, feel, or endure. Because it would have been too hard to continue with day to day functioning, those anxious, fearful, shaming, or angry feelings were pushed away to another part of the person's consciousness. Over the time, the person repeatedly pushesaway their painful emotional reactions. It becomes a coping mechanism that helps them function in a calm, adult-like, unruffled way.

However, at times, something happens that reminds them of past painful situations that they have bureid. The reminder is a trigger. This trigger causes the part of them that they have buried, that is holding onto all that anxiety, need, pain, etc. to have an emotional outburst. Because the person does not normally react this way, when it happens, it is experienced as "not me." The person has pushed that part of themselves away for so long that it is not recognized as a part of "self." Or, if it is recognized as a part of "Self," it is usually experienced as an unwanted part of Self.

Usually the person has learned to deny that part of themselves because they were told growing up that it was wrong to feel angry, or it was shameful to feel needy, or whatever. So when that "part" shows up, the person can feel really negative or even hateful toward that part of them. Part of the job of therapy is to help the client learn about those parts of themselves that they have disavowed, realize that there is nothing bad or wrong about about having feelings, and heal the pain that has made the person keep that "part" at arm's distance. The more a person can learn about their parts, accept them, and deal with their memories and feelings without pushing them away, the more the person can feel like a whole integrated person.

At least that's my understanding of it.

This is not, though, an explanation of "parts" as experienced by somebody with DID. I'm referring to parts as experienced by someone with BPD or Complex PTSD, (In DID, the parts are more fragmented, the person is usually not aware that they have parts, and they "lose time" when a part takes over.)