htaccess Redirect loop problem.

I hope I can explain this right! haha.

All my content lives in this dir

root/__us

but I want the urls to look like this

mydomain.com/us/

instead of

mydomain.com/__us/

problem is I can fix that, however for some reason whenever I have a dir like this

mydomain.com/us/about

without a trailing slash, it redirects to this.

mydomain.com/__us/about/

#------------------------------------------------
#RewriteEngine On
#ReWriteCond %{HTTP_HOST} ^www.mydomain.com$ [NC]
#RewriteRule ^/us/(.+)$ %{DOCUMENT_ROOT}/__us/$1 [QSA]
#------------------------------------------------

#------------------------------------------------
# TURN OFF WWW
RewriteEngine On
RewriteCond %{HTTP_HOST} ^www\.mydomain\.com [NC]
RewriteRule ^(.*)$ http://mydomain.com/$1 [R=301,NC,QSA]
#------------------------------------------------

#------------------------------------------------
#REDIRECT ALL US URLS
RewriteEngine On
ReWriteCond %{HTTP_HOST} ^mydomain.com$ [NC]
RewriteRule ^us(.+)$ /__us$1 [QSA]
#------------------------------------------------

#------------------------------------------------
#add slash to url on us dir
RewriteEngine On
ReWriteCond %{HTTP_HOST} ^mydomain.com$ [NC]
RewriteRule ^us$ http://mydomain.com/us/ [QSA]
#------------------------------------------------

Can you help me figure this out?

Thanks!