I am sorry your sisters party was not enjoyable for you. You mentioned you may end up shooting everyone some day. It sounds to me that by holding your depression in, and not dealing with it, your are, in actuality, harming yourself.
I would like to support you by saying you should look into counseling or therapy. I go to therapy, and it has greatly helped me. Even though I am still dealing with depression, I no longer want to die, or feel like I am going to explode from frustration.