So I'm just after an argue with my GF and I certainly feel like one. But I let the people of reddit decide what is what.

The background: I have a very close group of friends, some of them I know since I was too little to walk. They are very close to me and I really like time with them. My GF has a little bit rough relation with them, but she doesn't mind me meeting with them.
Unfortunatly there were situations, when I prefered spending time with them, rather with her (usually she and I had plans, my friends had plans and I wanted to fit into their plans). 
After a situation, when I was at the party on the other side of the city and she had a 30min nausea and nosebleed problem (I would get to her in about 20-30 min, and I thought that she would feel better soon) I promised her that I will make sure that she will be my top priority.
I also made a mistake of being unprepared for her birthday at the start of this year, what was a big FU for me.

The situation: my close friend will have his birthday this month. I was invited today, and I told my GF that I want to come. This lead to the argue I mentioned before. We wanted to come back to our homes for the weekend that the birthday would take place, and we would probably spend time together. It was clear, that I would have to stay in order to attend this party. She got mad at me, because I promised her that she will be the most important to me.
She asked me why I want to make his (my friend) birthday good, when I ruined her birthday, why I broke my promise so soon, and why is she the "second one important".
I told her, that he is very important to me, and It's his birthday in comparison to just coming back home to spend some time together.
I promised her that it will be the Last situation that they (i.e. my friends) will be more important than her.

Note: she has to go back as she has some students she must teach to on that day.