You have to store the submitted value for item_type, even it is the default "" value.
Looks like you might be using $cat1 as the variable that stores that value? I'd do something like this:
$item_type = $_GET['item_type']; // user submitted item type or just '' when page 1st loads
Using the same name for the variable as the form name has proven in the long run to save my sanity.
then change the condition statement like this:
if($drop_down_1['item_type_id']== $item_type){
and use
selected="selected"
to indicate the selected option.
And take the BR element out of the options list