I don't know about constipation and Endo, but if you haven't tried yet, I recommend you to try some natural treatments: drinking prune juice and taking Normalax. I found both to be very effective in my constipation.

I also have been struggling with constipation for two years now. I’ve taken miralax, exlax, dulcolax, prune juice, exlax, enamas, pedialax, and go litely none of them have really worked for me though but those were all recommended to me by my doctor if you haven’t tried them yet. i have surgery scheduled soon to check for endo and i also struggle with constipation so it could be related and i’ve read that endo can cause chronic constipation. I would also recommend a colonoscopy if you haven’t had one of those done yet.