Turkish Airlines Ticket Change and Cancellation Issue

I wanted to change my round-trip ticket purchased from Istanbul Airport, but since I couldn't view flights from Sabiha Gökçen, I started by canceling the outbound flight and paid ₺450 for this. However, I didn't know that I would forfeit the outbound flight. The ticket was changed to a one-way ticket. When I couldn't view the return flight on the THY app, I had to purchase another round-trip ticket. Upon checking the previously purchased ticket, I saw that the return ticket was still in the system, so I canceled the return ticket as well. I received a notification that the amount, minus a deduction, for the return ticket, would be credited to my account. No payment was reflected in my account for the outbound ticket. My intention was not to make it a one-way ticket; I wanted to cancel the flight and buy a new ticket because I couldn't change the ticket. I received a negative response regarding the resolution of my issue. I was told there was no problem with the app. I ended up paying for an extra ticket and ₺450. How do I forfeit my rights when the ticket is converted to a one-way ticket, which was not what I was trying to do?