You sound like me Double Trouble. Although, I have had many blood tests for my thyroid and they've all come back clean. My parents didn't believe in mainstream medicine, they were big on homeopathy so I never saw any doctors nor went to the hospital for my constipation. I do remember crying from the pain of going to the bathroom and having to sit on the toilet for 2-3 hours just to complete a BM. As a kid I'd only have a BM once a week or sometimes only once every two weeks. My mom took me to colon hydrotherapy a few times but it was expensive so we stopped going when it didn't provide a long lasting cure.
I discovered senna when I was 16 and have been using it for the last 15 years. I do drink lots of water (8 cups or 64oz.) but it doesn't seem to matter what I eat. I've tried many diets but everything just ends up getting stuck in there no matter what it is. Senna is the only thing that pushes it out eventually. PEG formulas, fibre drinks, magnesium, lubricating powders, and moisture drawing formulas don't work for me. They all just give me diarrheoa that flows past the compacted blockage and dehydrates my bowel.
I also have a set schedule for "bathroom time". I allow 3-4 hours every morning after I wake up for my bowels to work. During that time I drink lots of warm liquids, I sit in a posture that relaxes my back muscles, I focus on calming my mind and pushing away the stresses of the day ahead. During my morning nothing else exists, it's my time. Sometimes my bowels work and sometimes they don't, but at least they got a chance. Needless to say, I get very irritable if my morning gets interrupted.
I have tried enemas and glycerin suppositories but they don't reach high enough in my colon. My blockages are always quite high up.
Because I never saw any real doctors as a kid I thought I'd ask about anyone else having these problems since infancy and if they got any diagnosis that helped them. From what I hear though it seems like it's just something that I have to live with and that is greatly depressing for me. Having this does not allow me to have a life I want to live.