Add another variable to your code which will store your search start index and set value to 0 on program start:
private int searchStartIndex = 0
In your reset button code set value of searchStartIndex to actual count of ListBox.Item:
searchStartIndex = dropLog.Items.Count;
Next step is to modify your
for loop to start searching from specified item:
for (int i = searchStartIndex; i < droplog.Items.Count; i++)
I hope you find this useful :)