Ultimately, you need to do what feels right for you. Remember, you are not responsible for your girlfriend's attitude or behavior, and if she has insecurities, you are not accountable for how they may manifest. If you wish to attend the music festival, then do so and assertively (not aggressively) tell your girlfriend accordingly. Be open to shifting your perspective regarding 'asking for permission'. Yes, you are in a relationship but you are still an individual who is capable of making their own choices and decisions.