Life long constipation

I have had chronic constipation since birth. I am now 54 years old. I have ran out of laxatives to try. I went to a gastroenterologist a couple of years ago who did a marker test (nothing moved) and then attempted to do a colonoscopy but I told him beforehand that the solution did not clean me out so of course it came back inconclusive because it wasn’t cleaned out. He diagnosed me with colonic inertia and prescribed Linzess which sort of worked for a while then it started making me nauseous and not working. I started taking Miralax every day at the suggestion of a different doctor, that worked for a while and now it just makes me sick and doesn’t work either. I have an appointment with yet another gastroenterologist next month. I go weeks without a bowel movement sometimes 4-6 weeks and of course I feel miserable. Anyone else have the same issue?