Turkish Airlines Ticket Change Issue and Extra Costs

I purchased a ticket from Turkish Airlines for a trip from Istanbul to Montenegro, departing on May 30, 2024, and returning on June 3, 2024. Due to a change in my work schedule, I wanted to postpone the May 30 flight. However, I was informed that I couldn't reschedule because I used a promotional code during the ticket purchase. Consequently, I bought another ticket for May 31 by paying the fee. Since I was traveling one day later, I also postponed my return flight by one day to June 4, incurring additional costs. On the night of June 3, while in Podgorica, I was told during check-in that I couldn't board the return flight because I missed the initial flight. Despite rescheduling, paying extra fees, and purchasing a second ticket for my outbound journey, I couldn't use my paid return ticket and had to buy another return ticket for ₺27,000. This situation caused significant financial and emotional distress. Upon returning to Turkey, I plan to initiate all legal proceedings and pursue the matter. I condemn Turkish Airlines for this poor experience and their lack of assistance during my ordeal.