[onionknight]

What exactly does it mean, and what can be done to have a more stable, stronger sense of self?

[Living Dead Guy]

Sounds like you've been looking into BPD.

The coldly clinical version is this:

Some Borderlines have an almost eery chameleon-like quality to their social interactions: voice, gestures, clothing, opinions can change according to the person or group being idealized at the moment.

Lacking a stable relationship to self, it is common for folks with this disorder to exhaustively question every fundamental belief others may take for granted: their religious convictions, sexual orientation or preferences, moral precepts, goals and purpose in life. Unable to provide it for themselves, Borderlines consistently seek external validation of their self-value. Often, the assimilation into a group with strict guidelines and principles (military, religious or even cultist organizations) can substitute for this acceptance.

Another facet of this lack of identity is an observed tendency on the part of those with BPD to frequently quit jobs and/or change careers. In many ways, even an older person with BPD can be much like a teenager fresh out of high-school, unsure of their future goals and plans and reluctant to commit to one career path.

Many loved ones wonder whether the person with BPD in their life suffers from Multiple Personality Disorder (now known as Dissociative Identity Disorder), because their self-presentation can shift so radically from situation to situation. This has many causes, one of them being a tendency of those with BPD to dissociate under stress

The best description that I've ever come across is a poem. You can find the blog post here.
I’ve been here before, too many times to count.
yesterday I signed the papers that officially end my employment.
another door closes
I lock the door on it and throw away the key.
Combined with other recent events another version of me is gone
another identity
time to reinvent myself again
who will I be this time
where will I go, what will I do
unstable sense of self and identity
never knowing who I am
looking back at versions I don’t recognize
who was that person?
where is she now?

Here is another good description from HealingFromBPD. They have a number of very good posts.

My father was the type of person who could be loving and kind in one moment, but just the right words or actions, and you never knew what these were on any given day, and he would, without warning, become very angry and dangerously violent. As a child, I subconsciously learned how to "be" around my father. The "me" around my father behaved very differently from the "me" I was when I was with my teachers at school, or even with my mother.

I learned some to behave with my father in ways that would reduce my risk of being hurt by him. In essence, I learned to read him and then interact with and behave around him accordingly.

Evidently, this skill became somehow psychologically wired within me, and it wasn't until I started DBT (Dialectical Behavior Therapy) that I began to see how this contributed to my lack of sense of identity. I spent all of my time reading and responding to people in a way that I believed would cause them to accept and like me, that I never really developed my own unique sense of me. I'd see glimpses of it, but if I thought I had an opinion about something, then engaged in a conversation with someone whose acceptance, love, or approval I desired, and their opinion was different from mine, I instantly adopted their point of view. The interesting thing is that I didn't even mind. It was as if it didn't matter. I had no sense of attachment to any idea or belief. I just wanted to be loved, accepted, and to feel safe. Those were my priorities, and they served me well as a child.

As far as creating a stable self image I would say doing DBT, but I'll leave that to others as I'm the last person to give advice on that..