AITA: Annoyed at girlfriend because she will not miss her nephew’s 2nd birthday to celebrate my once-in-a-lifetime achievement

AITA? I (29M) will qualify into my chosen profession on 1 September this year. This is a goal that I have worked towards for over 10 years. On New Year's Day, my girlfriend (26F) of 6 years and I were planning out the year together.

I made it clear that I wanted to go on holiday to celebrate my qualification and that we should earmark the first week of September for that. My girlfriend's response was that her nephew's 2nd birthday is that week and that she won't go on holiday because of that.

My view is that it is only his 2nd birthday and, so long as her brother and SIL don't mind that she wouldn't see him on his birthday, there is no issue with going away. Additionally, he will (hopefully!) have more birthdays, whereas this is a once in a lifetime occasion for me.

My girlfriend flat out refused to even consider going away with me and just said that she would not miss her nephew's birthday. She wouldn't even consider asking her brother and SIL for their opinion.

It made me feel like I was not a priority and that she didn't really care for my accomplishment. She was very dismissive.

I could always go on holiday the 2nd week of September but that feels less special.

Am I overreacting? Should I just get over it? AITA?