It's a sad one to read, obviously if it's true and I'm thinking it possibly is as I know a guy similar to this, visits a WG once a month, takes her shopping before, turned into a a friendship in his eyes, and a regular in hers, sending money up front and only seeing one girl ever sounds too familiar.
The reality is she in it for the money only, it sounds like things have progressed further than being able to "work off the debt" you might want to call the police with your bank statement as proof embarrassing as it will be, probably the only way your getting any money back now or deep breath and walk away I don't think there's any middle ground.
Or some of the lads here use your credits and mail you a cheque? Hehe
Deep down you must be feeling anger at yourself for misplaced trust, you know you've been daft, lesson is don't do it again !!!