What you're feeling is in response to feeling betrayed and disrespected. Your feelings are normal. What is not, you letting them destroy you. You really are neither responsible for their actions nor for what they decide to do in the future.
Try to remind yourself that if they are having a good time, why should you suffer because of it? Apparently they disregard your feelings. Remaining angry you hurt yourself on top of them having hurt you. Show them that you're strong and try to focus on yourself and what makes you feel OK.