Yafay
assume that your website name is www.test.com
your shopadmin is www.test.come/shopadmin.
open your website admin panel.
select menu
Preferences -> SEO & URLs
find in this page Set shop URL lable
here is,
Shop domain : test.com
SSL domain : test.com
Base URI : /
now save it then first clear your all borwser data like cache,cookies all
then borwser open test.com
then try to login.
I use that in my website and it works good so hope that work also in your sites