You should upgrade, the current version of Ghostscript is 9.18 and 9.19 will be released very shortly. Each of the interim versions includes fixes to the txtwrite device.
Although it is true that PDF files do not include blank lines, the txtwrite device does have a mode whereby it will attempt to produce a reasonable representation of the original layout by using spaces and blank lines in a text file.
This is the default action in the current version of txtwrite, so you ought to be getting this already, unless you have selected a different TextFormat.
This mode is highly heuristic, easily fooled, doesn't cope well with superscripts, subscripts, significant point size changes and possibly other attributes which make the layout difficult to reproduce. Obviously without seeing your input file, there's nothing more I can tell you.