Angry at girlfriend

I'm a bisexual girl who currently has a girlfriend. We've been together for a few months now and she recently told me that she's polyamorous. I explained to her that due to my BPD and extreme jealousy issues, I wouldn't be able to stand seeing her with someone else, and she said she'd be fine with that, but I still can't stop feeling angry. I just feel completely betrayed. This has led me to have a breakdown and cut a few times. Am I just not good enough for her??? Even though she's saying that she loves me and won't leave me, I just feel so inadequate. I don't understand why she feels the need to have someone else and I can't stop feeling awful about this.

Do you maybe feel betrayed because she did not share this with you right from the start? If so, it is understandable. I am of the opinion these things should be discussed before a commitment takes place - so that everyone knows where they stand in the relationship.

How are you doing now?