So after reading your post I have a few thoughts:
1. You're doing nothing wrong and you should not feel guilty for doing what you do. If she has issues with you doing other people's weddings, it's her issues and not something you need to feel you caused.
2. As for her hurt, she is lashing out and perhaps she is not able to handle being in a triad; did she not realize that short of bigamy going into a triad with a married couple meant there was not really chance of her marrying your husband. By no means should you feel guilty for sleeping with your husband, being married, or performing ceremonies. Your girlfriend is way out of line putting all her hurt and issues on you.
3. If my wife's boyfriend ever, EVER said anything even remotely close to what she has said to you. He'd find himself not welcome in my house until he damn well apologized for it.
Ultimately I can understand your emotional exhaustion, but you have to realize there's very little you can do to alter her perceptions of marriage and your relationship. Live your life and leave her issues to her.
Polyamory is wrong! It's Multiamory or Polyphilia. Mixing Greek and Latin roots? That's wrong.