Welcome to our forum.
Emotions are not always rational; sometimes, they are trigger-related. I'm sure you'll be able to learn much as you check out our various threads and boards, and post your thoughts, questions, and concerns.
You're most fortunate to have such a supportive girlfriend. It sounds like the two of you are well-matched with each other. If another poly situation comes upon you in the future, you'll be prepared (as prepared as any of us can be).
Hope you find this to be a beneficial site, as I have.
Love means never having to say, "Put down that meat cleaver!"