I think the last part you said may be the key to this:
I don't get to see the success page
Since this code should be running on the success page if you do not get to it the sessions will not be cleared. You would not want to clear out the sessions before you get to this page, only after you get to this page.
You should try closing your browser and run another test knowing if you do not get to the success page the session variables will not be reset. Once you go through a transaction and get to the success page the variables will be reset and you will be able to test it again.