How was it at the doctors?
I have a theory:
gastritis = inflammation = immune system allowed to run wild i.e. it's not being dampened by cortisol ( = anti-stress hormone produced by adrenal). Caused by tired adrenals which is very common in people with prolonged illness.
Cortisol also makes the stomach acid acid. If it's not acid enough food will make you nauseous, especially fat foods and proteins. Heart burn is often a symptom of this (food stays too long in the stomach and bubbles up back upstairs. It's not being allowed to exit because it's not acid enough.).
HCL, diluted vinegar and lemon juice will help with this. Anti-acids will act counter productive.
BUT I don't think an inflamed stomach lining likes a shower diluted vinegar much...
If this theory pans out you are between a rock and a hard place. I'm so sorry. Also it's not the cause of your gastritis, just a co-factor.
Aiding the adrenals will help somewhat. This is done with taking proteins (chicken stock and egg yoke are the best. Also for me: Multigland by Biotics), vit C, salt, rest and de-stressing in whichever way works for you. Just loafing about in your comfy bed and stretching and yawning like a lazy puppy or kitten is a very good destressing method.
You can resort to Mindfulness or Zen meditation if you think puppies don't hold the secret to a low stress life.
I take hydrocortison for adrenals but this is a drug and you need a good doctor and yours doesn't sound too good ("hello, it's IBS, good bye")
I also take HCL or lemon juice with all my food. But I don't have gastritis.
If your poo is lightly coloured it means not enough bile is secreted. Bile is only secreted when the food in the stomach has been made acid enough. So the colour of your stool might give an indication whether your stomach is acid enough.
Best of health to you. Hope the doctor had something good to offer.