I so understand feeling like you need to talk to someone who isn't involved in it. Prior to entering our triad almost seven months ago, I told my husband any thought I wanted to without hesitation. Now, there are times when it's absolutely necessary for me to pull back and think things through, and decide... is this really something I need to talk to him about? Or can I resolve it within myself. Talking through a problem has always been SO helpful to me... but when the problems involve him, it's a lot more complicated. Add to it that they involve his relationship with another woman, and well... he feels badly. I don't want to make him feel badly if I don't have to, and so I come here to parse my thoughts.
I also get the self-esteem thing. I *thought* I was in pretty good stead in that regard, after years of struggling with body image issues (thank you Intuitive Eating), and having such a tulmultuous childhood that everyone who I should've been able to count on abandoned me or failed to do what I needed as a child at some point. Turns out, I was only feeling secure because there was no reason not to... meaning, I was the center of my husband's universe. He's NOT a social person. The few guy friends he had when we met all got married to controlling, manipulative women who didn't let their guys go out alone, and he just never really made new friends. He is perfectly happy that way. I am more social, but all the same, he's been my best friend for years and the only person I have basically no secrets from. He knows my darkest secrets, and loves me still.
Suddenly, I wasn't exactly the center of his universe anymore... at least not alone. I was sharing the spotlight, and what I've learned is that to some extent I'm a brat. I don't mean to be, and I suspect it comes from all those years with no one really loving me... and then to have so much love and be the sole focus of someone's life, well... I got spoiled.
What I have to remind myself of is that, I have MORE love, not less. I may have less ATTENTION, but that's not the same thing. Now, your situation is very, very different because you are not involved with the others in your partners' lives... and I am all but certain that is something I could *never* handle. I love him, and I love her... and because of that I want them both to be happy, even without me present. If I barely knew her, I'm honestly pretty sure I'd want to kill her... but that's me. Everyone is different.
The key is to figure out what you need in order to be happy. IF - and I'm not saying this is the case, but IF - you really can't be happy just because he's happy, that doesn't make you a bad person. It just means that's part of who you are. What would be bad, though, would be to lie and pretend it is something you can handle if you genuinely can't.
Having to sort through issues is part of any relationship. With so many side relationships involved, that's hard. It's also possible this other woman might be tremendously uncomfortable being friends with you. It's hard enough sharing my husband with the woman I also love and keeping boundaries in line... it has to be trickier to balance that when you and the woman in question aren't also a couple independently of him. She may be okay with the concept of you... the reality of you, however, may be another story. So, hard as it is, you may have to accept that she can't handle a friendship with you... and maybe THAT is something you will find you can't accept. Maybe if you could know her and feel she's worthy of sharing him with you, that would help ease some of your concerns. I am not sure, but I do know trust is paramount and if I were in your shoes, I know that not knowing her would be very hard for me because I could never trust her to not try to steal him away or change things. That would be something I'd definitely struggle with.
Now, all of these things are just how I'd feel... but I do think it's worth asking yourself if this is entirely about low self-esteem or if there is a fundamental difference in how you see his outside relationships. Just because HE can be happy when you're happy with someone else doesn't mean you can or even have to be. You are you.
So, above and beyond everything else, try to be as brutally honest with yourself as you can be... and try to separate, hard as it is, the insecurities from what *might* be simply differences in how you process these other relationships.
I could not handle my triad if we were not poly-fi. I relate very closely to things I've read by Mono on this forum, where I am emotionally committed to my lovers. I have one of each, and that's all I want. I don't think I would be very good at juggling more than these two, and more to the point, I don't want to.