Turkish Airlines Flight Delay Cost Me a Lot, Return Flight to Riga Canceled

I wanted to bring a very confusing issue to this platform because I received illogical explanations from THY officials. On June 18, my Riga - Istanbul - Denizli flight's Istanbul - Denizli leg was delayed by 5-6 hours due to THY. I wanted to change my flight to Riga - Istanbul - Izmir instead. I thought I could travel to Denizli by bus and avoid a long wait in Istanbul. However, THY only allowed changes keeping Denizli as the final destination. Interestingly, they would make me wait 5-6 hours in Istanbul and then send me to Denizli. So, I changed my Istanbul - Izmir ticket to June 20 and adjusted my Riga - Istanbul - Denizli flight to this date as well. I flew Riga - Istanbul - Izmir on June 20-21, paying an additional ₺7500. As a result, THY canceled my Istanbul - Riga flight on September 1 because I didn't take the Istanbul - Denizli flight. This is the hardest part to understand. My request is to have all my rights reinstated. Why cancel my return flight to Riga just because I didn't extend it to Denizli? I am a student and cannot explain this to my family.