There are literally oodles of discussions of this kind of thing on the web and detailed instructions on multiple websites. See, for example 'Split the single merged document into separate letters' at:
http://www.gmayor.com/individual_merge_letters.htm
IMHO, though, you'd do better to tackle the problem at the source. In addition to the alternatives in the link, see also:
https://www.msofficeforums.com/word/...documents.html