Cutting open the filter is good practice... but you need to go the next step and try to figure out WHAT is filling it up....
Do you have signs of algae ?
Do you use a clear 90 degree inline filter ? if so what does it look like ? Have you used any Diesel Doctor or Biocide or something like that in the last ten years ? Have you changed the other filters in your system ? Does your car have a filter in the fuel tank ? and have you ever changed it ?