I understand that your .txt file is already made of “lines”, i.e. some text terminated by an explicit line break (encoding depending on OS, but usually a single control char like U+000A LINE FEED or a pair U+000D CARRIAGE RETURN and U+000A).
When you paste into Writer, the line break is transformed into a paragraph break and you end up with lots of single line paragraphs.
This can be fixed provided either your plain text file has some structure or you modify it to impose this structure. By structuring the file (in your usual text editor), I mean you terminate paragraphs by a unique signature before the last line break, e.g. something like <*#*>
, making sure it never happens in your text.
When this is done, use you text editor find-and-replace function to remove all line breaks. Then replace all signatures by a line break.
Warning! Some editors won’t accept too long lines (my procedure outlined above will create a file consisting of a single very long line). A work around is to use an alternate strategy based on regular expressions if you know how to use them. There, no need for a signature. Terminate all paragraphs with a double line break (including the last one at end of file). Replace line break by nothing only if it is not a double line break. Make a last pass to replace double line breaks by a single line break.
Macro generators can simplify the procedure by doing everything in a single pass. But you still have to mark manually the paragraphs with a double line break.