2-3 years constipation. From you!

I'm suffering from constipation from last 2-3 years. I'm a vegetarian. I have tried every treatment literally. I have made changes in diet, did exercise, increased water & fiber intake. But I cannot empty the bowel. I tried Lactulose, Dulcoflex, Magnesia milk, Gut motility medicines, (all these were prescribed by the doctor) I have taken home remedies too. (Triphala) If I take coffee first thing in morning, I can go to poop but now it's not working anymore. And coffee gives me headache. When I take chai, Stool turns black n my headache. I take vitamin d & b12 regularly but no relief I'm fed up of suffering this much 😭 every consultation I take, doc says just increase water intake and doesn't find root cause. If you know what the root cause is, pls tell I'll take treatment from you