Problem with Chronic constipation

I have a history of hypothyroidism, severe endometriosis/treatment was a hysterectomy 17 years ago.I also have a hx of IBS. My thyroid hormone levels are stable. I eat like a rabbit(I poop like one too, LOL!!!!!!). I eat a large amount of fiber and drink almost every hour(nonalcholic).ONly prescription drugs I take are armour thyroid and my estrogen/prosterone troche. I make a coconut bread and I add psyllium husks,flax seed and chia seed to it. I eat it practically every am. I also eat a spinach salad for lunch daily with nuts and yogurt and eat a lean meat and veggie and more coconut bread at night. I do one cheat meal with a cheat dessert once per week. I am relatively active for my age of 60. I've been to a gastroenterologist and we have tried Linzess which didn't help at all. I take miralax, and a Super Cleanse tab one per day which has herbs to soften the stool and to stimulate the bowel and It barely works. I am at a loss. I have not tried Montegrity yet. I don't know if it will be helpful and the side effects scare me. I had a CT of my abdomen and it was ok, no obstructions. The gastroenterologist that I go to practically accused me of abusing laxatives. I would be impacted if I didnt use the miralax and the Super Cleanse. Diet/water and activity do not help. I had diarrhea from my late teens to the age of 36 when my intestines out of the blue just shut down completely. I was diagnosed with hypothyroidism, I was put on Synthroid but the constipation didn't go away. So if there is any gastroenterologist out there that can help me, please let me know.I feel defeated and worry about my health.