Booking.com Flex Ticket Not Actual Flex Ticket

I booked a KLM flight ticket from Amsterdam to Athens 14-25 March, via Booking.com under reference 40-872800119 plus accomodation. The reservation was made on 23 January, for a total of 374 euros, of which 50 euros was specifically for a flexible ticket option. Due to illness and a cancelled surgery because of this , I needed to return earlier. I was not able to change the flight myself through the system, so I contacted Gotogate customer support. Over about four days, they repeatedly told me that there was a “technical error” on their side and that it was not possible to change the flight, and I was sent back and forth between Gotogate/Booking.com and KLM. KLM informed me that they could not help because the agency had not actually purchased a flexible fare. Because the flex ticket did not work and no effective help was provided, I was eventually forced to buy a completely new return ticket through Booking, which cost me almost 500 euros. In addition, the delay in resolving this issue meant that my travel insurance only covered days without accommodation, so I incurred around 300 euros in extra accommodation costs for two additional nights while trying to sort out the return ticket problem. I am very disappointed that I paid for a flexible ticket option that was not honoured and that, despite contacting customer service, I received no concrete solution and was repeatedly referred to other parties or simply left without a response. I request a refund for the cost of the extra return ticket I had to purchase, compensation for the approximately 300 euros in additional accommodation expenses and the time and stress caused by this situation. I expect this issue to be investigated and resolved promptly.