Acid foods can cause canker sores (had to look this up - we call the little blighters "mouth ulcers"), but so can stress, injury, poor mouth hygiene (eg not brushing your teeth properly), a weakened immune system, gastro-intestinal disease or nutritional problems.
I have had gastric reflux disease for most of my life and also had mouth ulcers (sometimes multiple ones!). A few years back I went to the doctor about the GRD and was prescribed Lansoprazole which dealt with the GFD. I haven't had a mouth ulcer since.
When I did have mouth ulcers I tried every remedy known to man, from the home-made, such as rinsing with diluted hydrogen peroxide, salt, baking soda, etc., to over-the-counter remedies and nothing worked. Then I discovered the cure quite by accident. Don't sneer - it was honey applied onto the ulcer! It soothes the pain (and no-one who hasn't had one can have any concept of how painful mouth ulcers can be - I sometimes couldn't eat when I had them!), it cleans out the white gunk from the ulcer and it speeds up the healing. It really does work!
I'd suggest you visit your dentist as a sharp edge on a chipped tooth could be causing ulcers in your mouth and the dentist or the dental hygienist will also advise you on mouth hygiene.
A visit to your doctor to find out if there is any under-lying health problem would be useful. I know that visiting the doctor or the dentist in America is "a considered purchase" as they say on shopping television, and isn't to be undertaken lightly but in this case it could be worth the expense if it sorts out your problem. It isn't trivial if it's affecting your life to this extent.
In the mean time do try the honey. Any sort will do. It doesn't have to be the very expensive sort.