These two conversations are very different. They have different styles, different data, different “feel”, different implications. Swapping between them too quickly can falsely carry elements from one into the other.
Past-focused conversations are mostly objective (what actually happened; numbers, times, data). You can debate the subjective elements (was it good/bad/helpful?) from the data. You can also do real analysis; why did this happen? What were the chain if events that lead to this mistake?
Future-focused conversations are mostly subjective (what will we do? What will happen when…?). But you can be clearer on the desired outcomes (we all agree we want A, B & C). And you can plan out the processes.
So be clear, say, “Let’s talk about what happened last time…” And hold back all your “next time…” thoughts.
Later say, “ok, let’s change gears and talk about what we want in the future. Is there anything from what we looked atom the past we want to keep/change?”
Doing both at the same time is usually confusing and unproductive.