How can I help my therapist understand my issues?

I need your help.I'm in therapy and I really want it to work but during the past three sessions I have this issue that my therapist mentioned.I describe how I feel about my relationship with someone in my life but when I'm asked to describe in detail specific events my mind just stops working and my heart starts racing .So I keep talking, trying to help the therapist understand but it seems impossible to remember specific dialogues and events.Therapist said I'm probably not ready and this is therapy resistance.I feel ready though and in desperate need for help .How do I force my mind to stop blocking everything?This happens when I'm talking about certain people in my life and my relationship with them.