I have a New Years problem so.... not much time

I have been dating a girl since the end of July. I consider it serious. We are at the "I love you" stage. Something happened a bit ago that caused us to be "I dont know" for a few days. When we got back together we were lying in bed and I said "Lets start planning for Nw years!! so many things we can do". She said "oh I told my roommates I would hang with them". So her room mates are a couple. This was about a week ago.. maybe a bit more. I just said "oh". Frankly I was dumbfounded. Now NYE is about to happen.. she said "you are more than welcome to join us".... I'm very upset about this.. What does it matter if she said she would hang with her room mates during the 3 days we were having problems. Can she not say "I'm going to go out with my boyfriend"?? I don't know how to handle this cause I'm so mad. She even said.. "you are more than welcome to join us, their friend won't get off work till 10 so I'll be a third wheel till then"..... why are we not off planning our own NYE????? Im bad at being mad. I just always say the wrong thing and frankly I never seem to understand if I'm really wrong. Hence the first time ever I am turing to a chat room....... Help? Happy to give more details if asked

This may be in the pick-your-battles arena, and it is not a battle to pick. During the time you were on break she made plans to be with her roommates on NYE. Now you are back together and want to have your own personal NYE. But this lady already made plans and doesn't want to break them. Put on your game face and make fabulous plans with her roommates and her. If you want a special date with your special lady find something else to celebrate in January, and make those concrete plans with her for then.