Including Gallery

I'm using OSCommerce and Gallery. I'm not wanting to use Gallery to display pictures of my products, I just use Gallery to show events and pictures, but I want to be able to have my store "wrap" around it. Here's how it is now (http://www.arospeed.com/pictures/morossoaug). On the surface it looks fine, but if someone has something in their cart and then tries to access Gallery, the products don't show up in the cart. Also, when someone is logged in, the site uses a secure connection (https://) and when they try to access Gallery it comes up not found.

Right now, I have my includes copied into in Gallery's directory, though the rest of the site uses includes in public_html/includes. I know that this is the problem, but when I try to correct it by changing this:
require('includes/application_top.php');
into this:
require('home/arospeed/public_html/includes/application_top.php');

I get the error message:
Fatal error: Unknown(): The script tried to execute a method or access a property of an incomplete object. Please ensure that the class definition <b>navigationhistory</b> of the object you are trying to operate on was loaded _before_ the session was started in /home/arospeed/public_html/includes/application_top.php on line 313

And I have no idea what to do about it. Please, someone help!

Thanks,
Erica

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Give us your Gallery/webserver information to get a faster answer.
Get this information from the PHP diagnostic (in the configuration wizard).
Gallery URL (optional but very useful): http://www.arospeed.com/pictures/
Gallery version: Most recent, not sure exactly
Apache version: 1.3.28 (Unix)
PHP version (don't just say PHP 4, please): 4.3.2
Graphics Toolkit: NetPBM
Operating system: Linux
Web browser/version (if applicable):