Just like with Chron's disease, symptoms of colitis are bloody diarrhea. Again, you don't have these symptoms so I really don't think you have colitis.
There really aren't that many things that can possibly be wrong with your gut. You definitely don't have cancer (because the symptoms you are describing are not symptoms we see in people who have cancer), you most likely do not have colitis (either Chron's or ulcerative colitis) because you do not have the bloody diarrhea and the weight loss that usually accompanies colitis, you don't have a stomach ulcer.
You are scared to go get tests done because you have anxiety disorder which is not treated properly (you are under-treated). But there is not much else (or anyone else) can tell you until you get some tests done. None of the tests are scary and even if they diagnose a problem, the problem is not life-threatening by any means. If anything, it will help you so much. For example, if you have lactose or gluten intolerance, all you would need to do is avoid foods/products that have lactose or that have gluten. What is scary about this?
You should get an ultrasound or CT scan of your abdomen to look for gallstones, sometimes they can cause abdominal pain after eating but usually it only happens in right upper quadrant of the abdomen so i really dont think you have any gallbladder stones.
If you want to get better, you need to go and get tested. I am sure your tests will show nothing, which means you have IBS and just knowing that, will make you feel so much better. The more you worry, the worse the symptoms get.
I hope this answers your questions. Please, get the courage to see your doctor. You will be just fine but me saying that will not get rid of your fears. You will only feel better once you do the tests and find out that everything is ok.