I don't understand what is meant by "mindful" of the past. If someone is asking you to "reflect on a memory" they are no longer being mindful nor teaching mindfulness. The only way we use the past is to help us avoid pitfalls during mindfulness. If we know from the past that we have a tendency to stray towards a particular thought during mindfulness, then that knowledge of the past helps us to "thought stop" by recognizing the pitfall, and thus proceeding with mindfulness.

Thinking is the enemy of mindfulness.