Lots of teenagers feel like this at some point. Partly it sounds like it’s a confidence issue. If you have known them that long and they ask you to come out, they obviously like you! I have the same sort of problem sometimes because I feel that I’m just a clinger on or something, but you have to remember that they do want you around! If you don’t feel like going out all the time then don’t, it’s your decision and you are perfectly normal. But don’t let your self confidence hold you back- push yourself.