Forum Privacy Question

I have a question regarding privacy on this forum. I told someone outside this forum my username thinking they wouldn't be able to read my posts without registering and logging in but I've found that it's not true as I was able to read my journal without logging in. I want to change my username to secure my privacy. I looked in my settings and couldn't find a way to do so. Is there a way to change your username and if not, is there a way to set your privacy so that your posts are only visible to those who are registered? I know anyone can register but I don't think this person would go that far. I do think they may take a peek though. Thanks.