My best friend will be moving to another country for at least 3 years to take his master's degree. He told us he is not sure when he will be able to go back because he also planning to get a job there to support his studies and eventually build a life (we are not demanding anything of him, especially just to visit us). So for at least 3 years, the only sure way we'll be seeing him is online.

He has been my best friend since high school. This is a big deal because I am a very antisocial and introvert person, and he is among the very few people who I feel really comfortable being myself with. He even told me himself he wanted me to be there because I am his closest friend.

However, his farewell party coincides with my girlfriend's work event. It's like a super formal black tie mixer party for their current and potential clients, and she said she needs me to be her date. She has severe social anxiety, so I really want to be there for her (although I don't look forward to mingling with people because as I said, I'm antisocial as well). She is also one of the organizers of the event, so it's kind of a must to bring a date.

My best friend's farewell party has been set since last month, while my girlfriend only told me of her event last week. I have already confirmed to my best friend that I'd be going (I even told him I would bring my gf along), so I will essentially be canceling my attendance. I have also asked him if he could move the date of his party, but he said a lot of people have already confirmed, so I didn't push further.

Now, I am faced with what I think is an impossible choice, but I am leaning toward being with my girlfriend that night because aside from the fact that we'll get to spend quality time together, I don't want her to have social anxiety (or at least face it alone). I am afraid I might turn into that guy who ditches his friends for his SO though.

Would I be an asshole if I choose my gf's event over my best friend's farewell party?