These may just be canker sores. They're just open sores but painful and last a surprisingly long time if you do nothing. Since you mentioned that you get them from acidic things, they're perhaps caused by an opening in the mucus membrane, specifically a thinning in the membrane. If you accidentally bite inside you cheek, you can get open sores. Rinse your mouth with a hydrogen peroxide/water solution or warm salt water for at least thirty seconds. Do this at least twice before bed. Get enough sleep. Efficient wound healing will not happen you don't get enough sleep, and deep sleep at that, because that's when the body repairs itself, if I'm correct. Also keep your mouth shut after you rinse; don't open for anything, not even to yawn. Your mouth needs to be absolutely clean if you want those sores to heal within a few days, if you're lucky. It won't kill you, but even a gulp of air can repopulate those sores with micro-organisms and delay healing. Trust me, I've had my share of canker sores.