I'm sorry for your loss. So tragic
As a trauma survivor, I can agree with you, that you absolutely remember every moment with such clarity that will never leave you.
I believe OP (and other defendants) break down on the stand when confronted with facts that they cannot reconcile. Filling in the blanks in a lie is very difficult, and even harder to keep track of, even after only a day or two. When a story is true, you can recite moment by moment every time. A lie however is like a ghost, is has no substance. OP said he was emotional because he 'could not remember' when Nel asked about him getting around the fans. Once faced with the fact that his story wasn't adding up, he got frustrated and started crying.