You could go to Small Claims
court. It would not cost you that much. Ultimately, the credit card is in your name and, unfortunately, you are responsible for it.
There are forms in the Small Claims division of the Clerk of the District/Circuit court
's office you can fill out and go to court. If she paid you $50.00 for 8 months, that is $400, or around 25%.
If you do not have any receipts of what she paid, and she doesn't either, then you both are at a roadblock on proof.
You will need to go to the credit card company and get all the charges on it from inception. Look it over to see what percentage she actually did use this credit card because that is what a judge could look at. It will be hard to prove she bought gas, unless you get the signed receipts and can show she signed it. But, if she bought clothing that's another story.
You can call the small claims court to see what the charges are for filing a claim. You will have to appear for the hearing and put on your case just as an attorney does.
Good luck. And start keeping better records for yourself. It seems like you may need to defend yourself further in some of these situations.
If I can help further, please let me know.