That is the sad truth,
because nevertheless nobody want to pay more for your work: so many of yours already has to complain nobody needs them anymore...
What is left: Feed your selfconfidence and respect for yourself in doing good work, let them feel the difference between the job is done by you or themselves sometimes...
or frighten them with a suggestion of cancer or other heavy sickness. They won’t believe that no other can’t do your job with all that several agreements between you and your printing men and bookbinders and first of all with the single customers, but they will feel it, if you share ... ;-)
(and they argue already that a personal change must not fit at once: remember gordo’s last reprint-toon about the burnout, print him and fix it to your wall, to make them sensitive for your situation and get them slowly used to it... ;-) )
Ulrich