Hello. Thank you for coming to JustAnswer with your question, and welcome.
Your symptoms suggest that you do have other digestive problems in addition to acid reflux. Your doctor will need to do more tests, however, to determine exactly what the problem is. You may have gall bladder issues (right upper abdominal pressure and discomfort going to the back). These symptoms can also be related to stomach ulcers and/or inflammation or even problems with the pancreas. Inflammatory type disorders of the intestinal tract will cause you to see mucous material in the stool. And any of these problems will affect digestion, and energy levels.
So, the best thing to do is to continue to follow-up with your doctor, and possibly a gastroenterologist.
In the meantime, try to increase the fiber in your diet by eating more fruits, vegetables, legumes and whole grains. Avoid fat as much as possible as this may exacerbate your discomfort. Very small amounts of good fat are fine (olive oil, canola oil, avocado, oil from vegetables or nuts), but strictly avoid saturated fats.
If you cannot eat enough fiber, you can also try a soluble fiber supplement that you can get over-the-counter at your local pharmacy. There are many to choose from, and different forms (powders, wafers, tablets, etc). Just be sure to drink plenty of water with the fiber supplement.
Gastritis (inflammation of the stomach lining) can cause your discomfort, but would probably not cause the mucus in the stools. Of course, this would depend on what the cause of the inflammation is. If you also have inflammation throughout the intestinal tract, you could see mucus in the stool.
Inflammatory type disorders would include crohn's disease, ulcerative colitis (though your discomfort would probably be different and lower in the abdomen with these). Irritable bowel syndrome (the diarrhea prone type) can also cause more mucus production. Food intolerances can cause it as well.
There are other tests that can be done. Gallbladder disorders can be difficult to diagnose sometimes. For the types of tests that are done to diagnose gallbladder disease, see this link: Gallbladder Problems
Also, you might ask your doctor if you have been tested for H. Pylori, a bacterial infection that can cause inflammation of the stomach and other intestinal symptoms. You probably have been tested already, since you said you have had an endoscopy done. But check anyway, to make sure.