I was diagnosed with an Internal Rectal Prolapse last year. Before my dianosis, I would nearly pass out on the toilet, the pain was so horrenderous!!
I am glad to hear you have a good Colon/Rectal Specialist, most other doctors can't diagnose this problem without a test called a defecating proctogram. My Colon/Rectal Specialist advised 25-30 grams of fiber daily, fruits, veggies, whole grains, etc. He also advised Citrucel 2-3 times daily. And whatever you do, please don't strain on the toilet, this will make it worse! For me, the key was to not ever let myself get constipated! Try to do your business in 2-3 minutes and get it over with.
So far the fiber, water and Citrucel are working and as I have avoided the surgery, which would be a colon resection.
There is something else to consider, in some cases, the condition is treated for Pelvic Floor Dysfunction by a Physiotherapist, who will work to re-train those sphincter muscles which are contracting rather than relaxing. There are some old threads on this forum that are worth searching for, there is a lot of good information.
Good luck and hope you feel better soon.