I have antral gastritis too but only mid sternal pain. Pain between the shoulders may be gall bladder - worth asking about that if not ruled out already. And pain lower left side of back doesn't sound like gastritis either - well not to me, but I'm not a medic, just someone who's had gastritis for 2 years!!
It can also be caused by Helicobacter - worth getting tested for that too. I would go back to your PCP about the pains you have and ask them to verify that the pain is specific to the gastritis.
The only things that are specific triggers for me are tea, coffee and alcohol and overeating.
Have you tried Gaviscon?
New stuff: GERD, Recurrent cystitis/Overactive bladder
Lifelong stuff: Food allergies/intolerance, eczema, asthma