Every time I try to save my mod, I get the message "Write Error in TES File". I can't save any of my work, can't combine loaded plugins into a new mod... can anyone help with this or did I just lose a month's worth of work?

Can you save a new blank mod? Or is it just your WIP that is giving the error? No idea if this would help, but you could try with tes3cmd:

tes3cmd recover "my plugin.esp"