Self-esteem is actually the key to all aspects of life, not just relationships.
In my own experiences, I have found that the very need to be in a relationship was based on low self-esteem. (Not only for me, but also for some of the people I've been closest to).
In fact, I think our couple-focused culture PROMOTES low self-esteem. (By "our" I mean "modern society" or whatever, not polyamory).
Polyamory and non-monogamy in general require high self-esteem. Healthy monogamy does too.
But I also feel that traditional monogamy itself reinforces low self-esteem. You need someone else to "complete you"? If your significant other has feelings for someone else, that means you yourself are lacking something? The most significant moment of your life is supposed to be your wedding?
Self-esteem is raised by focusing on yourself. Increase your hobbies, your interests, your knowledge, your experiences. Go new places, make new friends, and above all, learn to enjoy being by yourself.
(That said, you can also become a better person and feel better about yourself by doing nice things for other people and becoming less self-centered).
I was at about a zero in self-esteem when I was in high school and at my first college.
Starting when I was about 21, my self-esteem jumped to about 6 (mainly because I became more self-aware and actually learned to like myself). Thereafter, it increased by a point every year. By 25, I was almost a 10, and I stayed that way for a while.
Last year, when I was 29, my self-esteem plummeted to about 2 when my ex-boyfriend blamed my for his sexual problems (among other things). Then it plunged to zero because I was so mad at myself for being so affected by my ex.
I've spent the past year crawling back from the abyss. I guess I'm at about a 6 again. I feel like I don't want to date again until I'm at least an 8.
Single, straight, female, solo, non-monogamous.