Feel your feet on the ground. It really is okay.
I agree with November that you aren't meant to take care of your partners above all else. Yes, of course, we want to act compassionately and take care of the people around us, but not at our own expense, and not by putting their needs before ours. Stop thinking of yourself as being selfish and that they are in precarious positions. They're not hanging off ledges on skyscrapers, after all. They are simply involved in relationship with someone who is involved with another. It's not epic or earth-shattering, really. As for moderating things? I don't know - what is it, a panel discussion? You take care of each relationship and nurture them to create a healthy dynamic of caring, love, and respect, but I wouldn't see it as moderating. Yes, you have to juggle your time and energies, but the people you're in relationship with are 100% responsible for their parts in it too. As I see it, the idea that relationships are 50/50 is bullshit - it's always 100/100 (or 100/100/100, whatever applies to the poly sitch).
And remember, if you've been honest with them, they are with you because they choose to be. You're not doing poly "to them," as if it were a weapon.
As far as feeling like you're going to fuck things up, well, I'm 51 and I still feel that way about almost everything in my life. Time to get over it and realize that you're doing the best you can and will sometimes make mistakes.
I think that you just need to take care to communicate as clearly and honestly as you can, figure out what you need and ask for or create it, and be kind to yourself. Enjoy what you got, honey! Also, when I have moments of doubt, fear, or or jealousy regarding my relationships, I try and remind myself that the people I love deserve to be happy and free, as do I, and that helps me act with integrity in order to support that for all of us - notice, not just people you love, but yourself as well.