Display non-printing characters to you can see what's causing the problem.
Either you have line feeds (hard returns) or paragraph marks at the end of
each line.

If you have line feeds, with a paragraph mark at the end of each paragraph,
then simply delete them using Find and Replace: search for ^l (el not one)
and replace with a single space.

If you have a single paragraph mark at the end of lines and two paragraph
marks at the end of paragraphs then, again using Find and Replace --

Search for ^p^p and replace with "XXXX" (or any string that doesn't
otherwise occur in the document). Then search for ^p and replace with a
single space. Then search for "XXXX" and replace with ^p.

You might need to do a little cleaning up before and after, for example to
remove superfluous spaces on either side of paragraph marks, double spaces,
etc.