you should leave it to use the post instead of get.
add the following code at line 1 of the page:
<?php
if(!session_id()) session_start();
if($_SERVER["REQUEST_METHOD"] == "POST") {
foreach($_POST as $key => $val) {
$_SESSION[$key] = $val;
}
}
?>
this will store the posted elements into session variables of the same name.
On the thank you page, instead of echoing the post:
<?php echo((isset($_POST["textfield"]))?$_POST["textfield"]:"") ?>
use the session:
<?php echo((isset($_SESSION["textfield"]))?$_SESSION["textfield"]:"") ?>