Should I be mad and pissed at my girl friend or just hold it in?

So I really don't know how to feel about this. I asked this girl out like a couple of days ago and she said yes. It was obvious she said yes without hesitation because we both love each other very much. So her birthday is coming up in like a month and she promised one of her good friends(which is a guy) that he could be her date to her birthday.......like 6 months ago. And this is the type of birthday where you dress up and stuff and dance and hold hands and escort the birthday girl as everyone watches (and in this case the escort is this "friend"). And my girl friend said " Don't get mad, but one of my good friend is going to be my date and I am not going to hurt his feelings." Plain and simple like that, like it's set in stone. And when she told me that I said "it's ok lol" but oooooooh deep down I was both mad and sad when I said that. And then I thought maybe I should cool it, if I really love her I'll understand. But then I thought wait wtf if this is a really "good" friend isn't he suppose to understand? . I feel like I have to choose 1 of 4 choices. 1.) accept and go to her birthday and watch as my girl friend holds hands and dances with another guy. 2.) Not go 3.) or just break up with her and say f it or 4.) Tell her I'm not going and wait for her to say for me to be her date and if not revert to choice 3.). And I need help picking. If she said she would blow off the guy for me I would tell her it's fine and to go with her friend anyway (cause I'm not an ahole). But the fact that she said she wont hurt his feelings no matter what really hurt mine. It seems like I'm number 2 ( I hope not lower). So if you read this and reply I will be very very thankful. And I did my best to not make spelling mistakes. So you might ask whats my question? Which option should I pick above and if you have the time, elaborate.