Turkish Airlines Ticket Change Issue for Malatya-Istanbul Flight

On March 10th, I purchased a round-trip ticket from Malatya to Istanbul at the THY sales office in Malatya Erhac Airport. The ticket was sold at the normal price, even higher than the online rate. Two days later, I wanted to change the dates and called customer service. They informed me that the ticket was promotional and couldn't be altered, directing me to the sales office.
Since I couldn't reach the Malatya airport sales office by phone, I drove 80 km to explain the situation in person. They told me, 'Your ticket is not promotional, but there is a system issue right now. Call customer service in a few days to make the change.' I drove back 80 km.
Today (March 15th), I called customer service again and was told, 'You cannot make changes to this ticket.' This was never mentioned at the time of purchase! They sold it as a normal ticket at a normal price! Due to unavoidable reasons, I won't be able to use my ticket. Who will account for the ₺3300 I spent? Look at the state of this huge company, THY. It's a shame.