Fantastic values

I can think of a few options depending on how literal "punching bag" is; if these friends demean and insult you, leave and get new friends without question. Those aren't friends. You don't owe them an explanation, and standing up for yourself will become a source of confidence that will aid you while looking for new friends. If these friends just make you the butt of jokes, perhaps you're timid/shy, and they aren't much worse than that - well, true friends would at least comment on your lack of confidence and try to help you get more. It would be valid to seek better friends in this case too. If you lack confidence and have self-deprecating humor that your friends perhaps go along with more than you would like, then if they are otherwise genuine friends, you could ask for their assistance in gaining confidence. However, it's worth emphasizing that for that last one, that it is only IF you may be excluding yourself a bit and your friends aren't doing much to exacerbate it.