Cech, who lost the Champions League final with Chelsea in May and has had a bad run of injuries, dropped a cross that led to Turkey's equalising goal.
"The only positive thing about this season is that it's over," said Cech, whose side lost 3-2 and now go home. "The season was unbelievable, just bad luck after bad luck."
Cech added: "When you make a mistake, you have to admit it. It was my mistake that decided that we have to go home.
"The ball was all the sudden too far away from me and as I tried to control it landed just in front of Nihat.
"Football is played by 11 players and, unfortunately, every time it happens that some do well and some do worse."