Turkish Airlines complaint: Turkish Airlines lost my luggage

I had a bad experience with Turkish Airlines. At the end of May I traveled from Panama to Istanbul (TK0904) and from Istanbul to Belgrade (TK1081). Upon arriving to Belgrade Nikola Tesla Airport I noticed that my luggage had not arrived with me. I immediately applied to the lost and found office.
I have a lot of important things in my baggage and this problem really upsets me. I was left without nearly all my clothes, my favourite books, my diaries with personal information. It’s really awful situation.
There is a pink large suitcase and a travelling bag. I don’t understand how is it possible to lost them…
It has been nearly two months now and my baggage is still missing. I’ve been calling and writing Turkish Airlines regularly during this time but there are no results, they just don’t care. I’m very disappointed and I have no confidence in Turkish Airlines anymore.