Product: Credit card or prepaid card

My name is XXXX XXXX, I am writing this letter to complain about the unfairness of the Investigation Department, Security Services at Citibank, NA / AAdvantage, in regard to a credit card, which was used by someone other than me in the XXXX XXXX XXXX store on XXXX XXXX. 
In XX/XX/XXXX, I went online to make a payment to above credit card. 
I inquired why my balance was so high, as I didnt remember buying anything lately, as I have Auto Pay and dont use the credit card much, nor do I get statements. I was informed that a purchase had been made on XX/XX/XXXX, at XXXX XXXX XXXX Store # XXXX in XXXX XXXX XXXX XXXX, for the amount of {$7000.00} for jewelry. 
I informed her I didnt make the purchase, she agreed to put an investigation on it. The credit card was cancelled, a new one was sent to me in the mail with the {$7000.00} charge on it. 
I received the statement copy, sent to me, looked over the charge on my card, which bothered me, but I thought it being a mistake, itll get fixed. 
On XX/XX/XXXX, I called Citibank N.A. /AAdvantage again, I spoke with XXXX XXXX for another investigation as I have never been to XXXX XXXX XXXX as stated on the statement, nor have I ever been or purchased anything in a XXXX XXXX XXXX XXXX. 
I received a letter on XX/XX/XXXX, from Citibank , N.A . Security Services stating, Investigation was completed, and the dispute is now closed, making me responsible for the charge because the charge card was in my possession. Making me the scape goat for this charge! 
On XX/XX/XXXX, I decided to do my own investigation, I called XXXX XXXX XXXX, after speaking with the fraud department, I was informed, XXXX XXXX XXXX do not require signatures on their receipts any longer and their videos are not available after 30 days, which is what I had asked about, to verify it wasnt me in the store making the purchase. I was also informed me that there were no notes nor any indication Citibank N.A. /AAdvantage had done any investigations on this incident, it appears no one has followed up on who really made the purchase. Im willing to take a lie-detector test or whatever it will take to prove the purchase was not done by me. 

Im XXXX years old, I live alone and live on {$1500.00} a month Social Security check. My husband passed away on XX/XX/XXXX, my XXXX adult  daughters are married and responsible citizens. All living here in Florida. If an investigation needs to get done on any of us, we are all willing to take lie detector test and whatever methods to prove Im telling the truth. 

This {$7000.00} charge needs to be taken off my Citibank N.A /Advantage Credit Card , as I did not make the purchase, nor did I authorize anyone to use my Credit Card XXXX 
Whatever needs to be done to prove Im not responsible for the {$7000.00} purchase, at XXXX XXXX XXXX. I will do. 
I dont know how this could happen, but I will not pay for something I didnt buy. Credit Card numbers can be inverted by someone not seeing too well and with all the technology out there, anything can happen, and it does. 
XXXX XXXX XXXX must have receipts showing a signature, videotaping by the store must have been done. XXXX XXXX XXXX is a prestiges store, how could they allow this to happen? 
{$7000.00} may not be a lot of money to XXXX XXXX XXXX, but to me its an overwhelming amount of money. Ill fight this until it is removed from my credit card, and credit reporting agencies. 

Im XXXX years old, I'm having sleep-less nights, stressing over my credit being ruined, if I don't pay this off and getting sick thinking someone's using my identity to make purchases without my knowing it. How can I protect myself against all the scammers in the world today?