I don't think any of this arguing is going to help the person who made this post; so maybe we should stop? Disagreements are going to happen, doesn't mean anyone needs to be up in arms about them.
I can sympathize a lot with the poster because I'm also brand new to this, and I've had similar feelings of jealousy, fear, and uncertainty when beginning my poly relationship. My boyfriend and his best friend from school fell very deeply in love, and for a while I felt extremely betrayed and tossed aside.
I still feel those feelings occasionally, when I'm in a particularly bad mood or I've had a long day, and there is an absolute validity to them - as someone said earlier, feelings are neither wrong nor right, they're simply felt. Which applies to your girlfriend as well - if she can't get her ex out of her head, then she just can't, and you need to accept that asking her to repress those feelings will cause her to be awfully unhappy. You do need to decide how you're going to live, based on both your feelings and reason, so you can't let every emotion that comes along unwind everything - however, something I've found is that while I can't hide or deny those "bad" emotions, (and by "bad" I mean those emotions that I've decided would hinder my ability to live the way I want) I can definitely keep them from affecting my decisions. When I feel upset or insecure, I either talk to my boyfriend and he reassures me, or I go off and write, draw, work out - whatever works for you to help you calm down. If your girlfriend can't help you calm down, or isn't willing too, then you need to either find your own way or accept that she can't be poly in a way that will make you comfortable being with her.
What I've had to do (hell what I'm still in the process of doing) and what you need to do is to let go of the idea that your feelings give you a right to possess her. And what SHE needs to do is realize that her feelings don't give her a right to treat you with such contempt - honestly, I can't believe the way she spoke to you. You need to tell her how much that hurt you and she needs to apologize, or it's going to get worse - that's just plain old relationship advice from someone who maintained a wonderful monogamous relationship for 3 and a half years.
I can't stress enough the first and what I think is the MOST important thing that I learned when researching polygamy... communicate! (Thanks to all the super-awesome people who have said that repeatedly on the forums, because our emerging poly relationship never could have happened without constant communication.) It's good that neither of you are hiding your feelings, but fighting about them isn't going to make it better, you both need to swallow your pride and anger and just talk about boundaries. And you might find that you do have clashing hard limits, or you might not.
I know I'm speaking from what would probably be the opposite side of the situation you're in, but that's what I've learned from being in a similar situation. I hope that any of that helps, and if it doesn't then feel free to ask, I have a bit of a headache right now so I may not make complete sense haha