I know how you feel - I've been dealing with IBS for 40 years & it's taken a long time to get it right. I've been through all of the testing & the doctors. You will need to eliminate or adjust the amount of your intake of several things from your daily diet to find tolerance levels. The doctors can eliminate any major health problems that may be lurking & they can point you in the right direction in regards to things to look for, but in the long run, if all other health issues have been eliminated, it really come down to you. It will be up to you to do the testing of foods and vitamins to make the necessary adjustments to your diet.
Things to look at: dairy products, sugar, alcohol, caffeine, carbonated drinks, wheat (gluten), very greasy foods, & yeast (like sourdough bread - this was a big problem for me at one point).You should consider eliminating all of these for a while & then slowly re-introducing them back into your diet. Why? You need to find out what bothers you. Everyone is a little different in what bothers them. You have to find out what specifically bothers you!
Things you might want to try: Enzymes like Primadophilus w/Bifidus (time released), Pancreatin. DGL before every meal (licorice - takes getting use to the flavor) And if things get really bad, take a charcoal pill after each meal. Coral Calcium - this helps me with my acid/alkaline balance. This saved me!! This is what got me started on the road to recovery. Best thing I ever did. (give it time to work) More fiber. This part is a little tricky because, from my experience, some things will make things worse, not better. I didn't like the flax seed, fiber pills & metamucil - for me, it required to much water. I use Chia seeds & Quinoa.
In order to get to a point of feeling better, you will need to be patient & persistent. Again, what works for one person might not work for you. Start by eliminating all of the things that you think are making you sick and stay away from them. (I personally go 2-4 weeks before going back to any foods) Don't introduce to many new things at once. Try 1-2 things (like vitamins) and see how you react. Eat smaller meals. Don't eat until you feel full. You might still feel hungry, but your body will adjust over time. Find healthy snacks for between meals.
There is so much more I could say, but this is a start. From what you have mentioned, I would start with some DGL & Charcoal pills to try and get a more immediate relief. I would also try come Coral Calcium pills (I have found Source Naturals to be very good) Hope some of this helps. Feeling somewhat normal will take time. Be informed. Use the internet to gather information and don't be afraid to test out new ideas. Good luck!