Turkish Airlines Denied My Ticket Change Request

I realized that I had accidentally purchased a non-changeable ticket category for our family trip abroad through the Setur agency. On the same day I bought the ticket, I contacted both Setur and Turkish Airlines to request a change in the ticket category, but I received a negative response from both. I clearly explained that it was a mistake, that I am willing to pay the fare difference, and that I would be seriously affected if our visa gets rejected, yet no one is willing to assist. I even asked if the ticket could be canceled and reissued with the correct category, but they refused that as well. Is such a rigid attitude really acceptable? I sincerely hope someone with empathy can help me resolve this issue.