adding www to site address (URL) breaks cookies

Hello all,
I just installed a blank wp 3.8 in http://mydomain.in/site directory on a shared hostgator server.
I followed instructions for Giving_WordPress_Its_Own_Directory and everything is workng fine and I can access the site on http://mydomain.in

Now if I add “www” to Site address in General settings panel and make it http://www.mydomain.in Now when I click on “visit site” in admin bar, it takes me to http://www.mydomain.in BUT I am no longer logged in , hence I can’t see the admin bar. This is very problematic as admin bar makes our life very easier.

I have checked for it in forum but was not able to find any solution. Somebody suggested to add

@define( ‘ADMIN_COOKIE_PATH’, ‘/’ );

in wp-config file, but was not helpful.
My domain is http://pratikriya.in.

Please help