How do I deal with my girlfriend being away on vacation and us not being on such great terms?

Hello. When there is a situation where you and your partner are not on great terms and one of you goes away without the other, it can cause anxiety with some intrusive thoughts. Many people question what their partner is doing, and these thoughts being overwhelming. Without knowing details about your relationship, I can only speculate what is most troubling about this situation.

You mentioned that she agreed not to take the trip, but went on it anyway due to you acting out on your emotions; this makes me wonder if changing her mind and actually going on the trip was out of spite or rather needing her own space. You can not always heal in the environment which broke you, and some times space is a healthy thing to take. If she left out of spite, the relationship is not healthy at the moment and the communication is poor. 

For you trying to deal with your partner being away, it is important to allow yourself to feel your true feelings; if  you feel mad, sad, anxious, etc, it is necessary to express that in a healthy way. If you are feeling sad, depressed or down, using exercise is the best way to create endorphins and endorphins make you happy. Exercise, whether traditional like at a gym, or more fun like dancing, is a great way to get that negative or sad energy out. You can try to journal your feelings but when you do, I always suggest not focusing so much on what you are writing, because the point of journaling is to just get the thoughts and feelings out of you. You can process your feelings with journaling because once it is all out of you, your head is less cluttered with distress and you can focus on one thing at a time. 

It would be a good idea to go out and distract yourself as well. Either independently or with friends or family. Do something that is good for you and that makes you happy, because you deserve to be happy even despite the situation. 

Healthy communication and understanding is the most important part of a relationship; when she comes home, having a discussion about how you feel about her going on the trip and about how things have been, is necessary. Use 'I feel' statements, because when you use feeling words, it doesn't allow the other person to feel attacked. I hope this helped a bit.