I try to hide the negative thinking whenever possible around other people, but you're probably right: it does show in some ways.
When you say I should focus on developing myself, you mean things like working out, getting hobbies, making friends etc?
I think I have at least decent body language, at least when walking or sitting by myself (when talking to someone it's a different story entirely). Facial expressions are perhaps the killer for me.
I've even gotten random homeless guys come up to me and tell me I look "angry" or "p1ssed", or that "you look like you're hoping wherever it is you're going is on fire". Not entirely sure why I get this: I think from years of being depressed, it just shows in my face