:mad: I bought this game from the gamestop direct download, and I played it for a long time, but recently, it said "my trial expired" after buying the whole game. I don't want to have to buy the game again, so what else can I do?

From the thread titled ►►► Please Read Before Posting ◄◄◄:
We can only provide support for CD versions of the game.
If you paid for a downloadable version and are unsatisfied with it, you may want to try to get a refund (http://forum.team17.co.uk/showthread.php?t=33495) and order the CD version (https://secure.team17.com/browse.html?area=detail&skuid=79) of the game (it will likely be much cheaper than the downloadable version).

Contact Gamestop since it was them who sold you the game. Then, get a refund and buy the CD version from Team17's online store (see CyberShadow's post above), and that will allow you to play online and update to the latest version of WA, which the Gamestop version won't.