Open the find and replace dialog (press CTRL+H).
Regular expression in the ‘Search Mode’ section at the bottom.
Find what field enter this:
There is a space after the comma.
This will also replace lines like
Apples Apricots Pear Avocados Bananas
Where there are empty lines.
If your lines have trailing blank spaces you should remove those first. The simplest way to achieve this is
EDIT -> Blank Operations -> Trim Trailing Space
TextFX -> TextFX Edit -> Trim trailing spaces
Be sure to set the Search Mode to “Regular expression”.