I'm sorry to hear about your troubles. I just had my first colonoscopy early this week, and I know now how they aren't fun. I have had problems with constipation, so I have had different types of fiber for awhile now. My GI recently told me to take metamucil, and work up to the highest dosage after a little while. It's supposed to be like a completely natural laxative because it's pure psyllium. This might work for you
I'm only 19, and it was pretty gentle on my system. I bought the flavorless brand instead of the orange flavor so I can mix it with anything.
Good luck with your scope and hope you find a good fiber supplement to use in the mean time.