Re: getting very discouraged - somewhat gross

To help digestion on an immediate basis I would take maybe a teaspoon of apple cider vinegar about a half hour before meals.
You should see a difference with this.
For some reason your body may not be producing enough digestive juices to get the job done.
Good hydration may help.
Have a goal of half of your body weight in ounces of water daily and add a little good Sea Salt of Himilayian salt to it.
The good salt is important for proper hydration and proper hydration is very important for digestion.
Your doctor looking at the colon could be a bit like a beer brewer looking at the surfaces of the inside of a beer keg to figure out why his beer isn't brewing properly.
Its the fluids that contribute to the action of digestion that is important.
Not sure the specifics on that but it could be an overall functioning of the bodies organs.
In order to rev up body functions you could look into "oil pulling" and Iodine supplementing,
Both of those should be beneficial.
Also be sure to consume good oils like olive, canola, hemp seed or coconut.
oils are necessary for glandular function and its the glands that control the functions of the body.
These are the immediate things that come to mind.
Of course there are a number of other remedies that improve the overall well being of the body that you will find on the different forums.