It's weird that your admin area is accessible by mysite/admin. Default silverstripe installations don't do this. Did you change something for this to happen?
The default behavior of SS is to make the admin area accessible by www.mysite.com/admin
I set .htaccess in this way:
and apache in this way:
Options Indexes FollowSymLinks
Allow from all
Alias /mysite "D:/_project/mysite_ss"
So then use htaccess to redirect mysite/admin to /admin