Unfortunately, if all the tests that you have had have come back clear, then I would think that you do have IBS and gastritis.
I don't know whether you know, that IBS is diagnosed on a basis of eliminating all other causes of you symptoms? This means that docs will perform all these tests to exclude such things as inflammatory bowels disease, this includes diverticulosis, ulcerative colitis and crohn's.
It is good that your endoscopies have been clear apart from gastritis and that your colonoscopies have been clear also. Have you ever been tested for Celiac Spru? If not maybe you should be.
Unfortunately, IBS can give you severe stomach pains with diarrhea/constipation, nausea and feeling full after eating. This is not unusual for people to get these symptoms. Also, lactose intolerance and other food allergies can play a large role in IBS symptoms. Dairy and wheat seem to be the main culprets so you could try cutting these out of your diet.
Are you taking Metamucil at present? If not, start taking this twice daily with plenty of water and it will help constipation and diarrhea by firming up your stools.
Try Acidophilus caps or eating some live yogurt to help re-balance the good bacteria in your gut. This could help problems with gas, bloating, nausea and stomach pains. Take them on an empty stomach and keep them in the fridge and you should start to notice a difference within a few days.
Drink plenty of water and try to cut out junk food, caffeine, alcohol and dairy. Eat a good all round fresh diet with fruit, veg, and carbs such as potatoes and rice.
If you have constipation then it might be worth taking a Magnesium supplement and if you have diarrhea, then try a Calcium supplement.
Aloe juice is meant to be good for soothing the burning pains of gastritis. Also, both Peppermint and Chamomile tea are very good for soothing a painful and upset stomach.
Have you tried putting heat pads on your stomach to help with the pain? If not, try this as the warmth tends to relax the muscles helping to ease a bad stomach ache.
Lastly, try massaging your stomach with some oil or asking your partner to do it for you. You only need to do this very gently and in circular motion all round your stomach. This is very good for breaking up pockets of gas that might have become trapped and these can make your stomach very painful. Once released, you will feel that your stomach is better. It is also very relaxing.