Product: Credit card

I travel for work, often for months at a time. So I receive physical mail sporadicly. Late last XX/XX/year>, I received an email from Discover that they were going to cancel my {$20000.00} credit limit if I didnt use my card. To use the card, I needed to have a new one sent to me. They wouldnt send it to my road address, just my permanent address in XXXX. My daughter and grandson live there and she handles my mail. I talked to a Discover representative, and to meet the deadline, my daughter, XXXX, had to activate the card. I told her to use it for a couple of grocery trips so I could maintain my credit limit. This was discussed with Discover, taking multiple calls to sort out. 
As far as I knew, everything was settled. 
In late XXXX or early XXXX I received notice that my credit score had dropped. When I checked, it was Discover tagging me for late payment. 
I returned to XXXX in early XXXX and seeing my bank statement, found out that an unauthorized person had been using the card and ran up {$8500.00} in charges starting around XX/XX/year>. At this point I had not seen or used the Discover card. 
XX/XX/XXXX, I called the fraud division of Discover to dispute the charges. They opened an investigation and I was told it could take up to 90 days to resolve. During this time, they had trouble accessing my account, because my name, email and phone number were taken off the account and XXXX XXXX name and information was on the account. I told them that I did not authorize him to use the card. Discover didnt ask me to file a police report, I assumed they were taking care of the investigation, like the times Ive disputed credit card charges with other companies.

I have a paper trail of most of the correspondence, including bank statements from Discover and emails with the XXXX Police. Discover wouldnt let me have access to their phone recordings unless I had a subpoena. 
We had quite a few phone calls starting the end of XXXX through XXXX. 
On XXXX call, they asked me to open a police report. On XX/XX/XXXX I filed an online police report and gave Discover the report number and XXXX Police Department info. 
( As an aside, my life partners XXXX took a turn for the worse in XXXX and I had to leave my job to return home for her care. She passed XX/XX/XXXX. ) During this time, I received another notice or XXXX from Discover stating they had declined my dispute and they considered it resolved, case closed. I called Discover and they told me they hadnt even reached the XXXX Police XXXX. 
On XX/XX/XXXX, I did a follow up call with the XXXX Police and found out my original online report didnt go through and I could file a report right then. Detective XXXX XXXX, fraud division, was assigned the case. I sent him all the bank statements and talked him through the timeline. He told me I needed a written statement from my daughter, XXXX, stating our original agreement. I have a copy of her statement. I told Discover the XXXX 's name, case number and contact info. 
Discover feels the case is closed and I owe them the balance on the card. 
I dont feel I should have to pay for charges I didn't make on a card that I didnt even see until arriving home in XXXX. 
I have been making payments on the balance, but with interest, the total amount has ballooned.