The turning point occurs during Gatsby's confrontation with Tom, the moment Daisy can't deny that she did love Tom once.
Explanation:
It is this moment in which it becomes apparent that Gatsby is not going to "win" the confrontation because his <em>hopes to erase the last 5 years can't be maintained.</em> It is impossible for Daisy to deny her past and to not accept it. It is here when we realize Gatsby has asked for too much and will not win.
