The best advice I can think to give is; take it day by day. Try not to overthink things and focus on what's right in front of you. There's a good chance you'll get used to things with time. I'm sure the initial shock of living by yourself in a new country is making things worse but it'll get better.

Your issues will be with you whether you're at home or in Japan. I really would encourage you to do your best to stick it out here. It might be the best way for you to confront your issues, even if it's very difficult. Try and go out and meet people and experience new things, don't sit alone at home. Try your best to embrace the experience.

Other people have overcome similar issues and so can you. Give it your best shot, that's all you can do!