paypal IPN help quick please...

hi
i have problem with using paypal IPN from my digital commerce site

when i process order process and complete it from my site then go to paypal site for transaction, every this is ok till here

when i click on cancel ( first screen of paypal ), it return back to my site error.php page. it is ok

the problem is: it expires all the session associated with my site last page(before going to paypal)

and also this problem occurs to other paypal IPN actions like success or IPN notifications

can anybody tell me why my session goes to expire and also guide me to what to do with PAYPAL IPN and perhaps a tutorial tooo

thanks buddy in advance...........