I have real problems with my wordpress

Those are my problem with my website:
It lately takes too long in loading when I try to log in, add a new post or any kind of activity that I do in my website.

Another thing, those are the errors that it shows me.

Warning: array_key_exists() [function.array-key-exists]: The second argument should be either an array or an object in /home/content/68/7982668/html/wp-content/plugins/addthis/includes/addthis_addjs.php on line 53

Warning: Cannot modify header information – headers already sent by (output started at /home/content/68/7982668/html/wp-content/plugins/addthis/includes/addthis_addjs.php:53) in /home/content/68/7982668/html/wp-includes/pluggable.php on line 876

PLEASE HELP ME!!!!! ??