I know that feeling. It feels as if you are the castaway, the backup friend; this feeling is horrible to experience because not only do you realize that you're replaceable, but that these people don't have any real interest in you. They use you, that's all. Let me tell you something though, talk it out with your friends, find a common ground and build up from there. No one knows what you're going through if you keep it quiet. I believe in you brother.