My fiance came out to me as poly four days ago, and told me she had asked someone to be her girlfriend. They already have plans of flying out to visit each other in April. I'm really struggling to come to terms with this, she is the only person I have ever been with, for a total of eight years now (with a two year separation which I know now was because she was having poly feelings and didn't want to hurt me). I am scared and so anxious, this is my future wife and this is all moving so fast.
You are right that your fiance is moving way too fast with this. Shifting a relationship from monogamous to polyamorous takes a lot longer than four days and one conversation. When someone tells their partner that they want to open the relationship, that should be an “I’m consulting you” conversation, not an “I’m informing you” conversation. Your partner doesn’t need your permission to be polyamorous, but when it comes to changing the fundamental terms of the relationship, that does require your input.
You have every right to ask your partner to slow things down. And honestly, if I was monogamous and my partner just flat-out told me they were going to date someone else and then went ahead and did it within less than a week, I would consider that cheating. You don’t have to be in a relationship on terms that are not healthy or fulfilling for you. I would put the brakes on this engagement and figure out whether this is a relationship that makes sense for both of you to stay in long term.