Question about chronic constipation?

I'm a 20-year-old, otherwise healthy female but I have had chronic constipation for my whole life. As a child I was given every laxative you can think of by my pediatrician but they all made me feel worse than not being on them. At different points in time I was on mineral oil, miralax, metamucil powder and wafers, milk of magnesia liquid and chewable form, senekot, and I'm sure I'm leaving something out. All of this was treatment from both my pediatrician and a pediatric GI specialist when I was about 7 or 8. Anyway, I have a bowel movement once, sometimes twice a week and it's always very large, dark, and hard. It's embarrassing but it's too large to even be flushed and has to be broken up first. And even then, it still clogs the toilet fairly often. I drink water constantly and I try to eat extra fiber but fiber only makes me gassy and uncomfortable but does not help with the bowel movements at all. I am slightly overweight so I know it's not a malabsorption issue. My Grandfather had ulcerative colitis and my father has Chrone's Disease and he had colon cancer. Both the opposite of my symptoms. I'm getting frustrated with this. Does anyone have any advice?