Originally Posted by hylierandom
So I guess my goal is to build up HER self-esteem until she either (a) feels secure with my poly-ness, or (b) feels special enough to tell me to get lost because she can't take my poly-ness.
The only person who can build up her self-esteem is herself. Her parents could have done it when she was a child, but that ship has sailed. If it's ever going to happen, the responsibility is squarely on her shoulders. There is nothing you can ever do to replace the hard work she needs to learn self-esteem.
You are her partner, not her therapist. What she needs is counselling. What she does not need is a partnership that fails to meet her emotional needs.
Another strike against your approach is that with her mental state (lack of self-esteem, fear of abandonment), a partnership with you is more likely to harm her than help her.
Self-esteem: She'll be asking herself, "Why am I not enough for you? What's wrong with me that I can't satisfy your needs?" It will never NEVER click that polyamory is about your needs as a polyamorous person, not her failure as a partner.
Fear of abandonment: She'll see you going on dates with other people as you looking for her replacement. She will spend every moment expecting you to come home and tell her it's over, that you've found someone better.
There is nothing you can say or do to change that, because she does not have a healthy mind. You can't reason with an unreasonable person.
The best thing you can do for yourself and for her is to let her go as a partner and continue to be her friend. You don't have to abandon her completely. Tell her that you need to be polyamorous in order to be true to yourself, but that you can see how much polyamory is hurting her, and that you do not wish to hurt her. Encourage her to seek help and stand by her as she makes the difficult life changes.