Is AUTHOR wrong in the post ? POST : TL,DR at bottom

So some backstory

We're both on a small vocal group, and we've been together for almost 10 years, and members of the group for 8years. The group is formed mainly on post-highschoolers and undergraduate people who have been doing music almost their entire life, but don't want to quit doing it after pursuing a  different career like engineering, lawschool, medicine, etc. We sing from academic music to the popular type. There are plenty more girls than guys. 

3 months ago all the male members of the group have had trouble going to rehearsals, myself  included. The reason ? We are all at different points in our careers, some starting university, some finishing up, some already working, and that sometimes can get on the way of singing. Noticing this, the conductor decides she is going to do the end of the year program with just the girls and build a program made to work with female voices. A cool way of handling the temporal absence of the guys. While picking the songs, the conductor asks my girlfriend if she could have a couple of Christmas songs arranged to work on the female choir, since she's been doing some self-taught composition, so they'd add them to the program. She agrees and delivers. They prepare during the three months for the concert. During this time she also finished and got her engineering degree.

Now, I'm finishing my degree on Computer Science and me and my friend are working on our dissertation/thesis, this is the last phase and we have to finish the project and deliver the paper this week. However, our project is quite big for an undergraduate project. When we started, our advisor, and also other advisors, told us it was a task closer to graduates. We dived right into the madness and went with it. Took also a part time internship at a software company.  Long story short, a year passed by and this is the due week.

The date for the recital, was last Sunday, late afternoon. She was really excited I was there to see her and the songs she arranged. I told her I would be there. A month ago I asked for a month permit at work so I could work full time on my thesis since there was no way I could finish working with them at the same time, they willingly granted it. Me and my friend are going every single day all day to the campus, Sunday to Sunday, for a month to work on it. Still managed to make time to see my girlfriend every Friday and Saturdays. We, myself and my friend, were at the university on last Sunday morning finishing the last details before collecting results, but something came up which could prevent us from finishing within the date limits, and my permit ended so I couldn't work on it the next day. I called my girlfriend and told her I may not be able to be at the recital since we had this problem, and needed to fix it, but will try to be there hoping she would understand. Stayed till late night at the university but we were able to fix the issue.

She was disappointed, and said she told me with time when the recital was. She's upset I wasn't there despite I explained why and now she thinks that her things are not important to me and that she always makes time for the things that matter but I couldn't make time to go to the recital.

Am I the asshole ?

TL,DR: girlfriend had recital on Sunday where she arranged the songs. My dissertation due date is the week after the recital, problem came up on Sunday and had to stay late night fixing it or else I might not be able to finish it and missed the concert. Girlfriend is now upset despite me calling her to inform her the problem came up and might not be able to go.