I solved my problem, and it could definitely help others too.

The problem was with cookies since my folder structure, since it was like this :

• mysite.com
• mysite.com/mypage.php
• mysite/wordpress/

the cookies were only valid for wordpress folder, and it's subdirectories.

In order to activate the cookies outside wordpress, I installed the root Cookie plugin, and everything works fine now.

P.S.

Do NOT change the configuration of root Cookie. Just leave the Domain Name setting as empty.