Total Commander

When i "select/unselect files", my new selections no longer save, as the buffer of memorized selection is full.

is there a way to flush all of them at once and start anew?

That should never happen, it should forget the oldest. The list is stored in the file historyPrefs.xml in folder
/data/data/com.ghisler.android.TotalCommander/shared_prefs
You can go there without root rights via home folder - system root.