You don’t describe what the mouth ulcers look like. If they are a small white ring surrounded by red swollen tissue, you may have canker sores which are caused by the herpes virus. (Not the same one that causes genital herpes.) Since your doctor is not able to suggest a treatment, I would suggest you get a second opinion from someone who can diagnose you what you have. Also, talk to your dentist about any mouth issues that may be causing sores.
In the meantime, to soothe your mouth, get some liquid Benadryl for internal use (not the spray on type for bug bites) and some liquid Maalox antacid. Mix small equal amounts of each (1 tablespoon) and swirl the solution around in your mouth. Spit it out. The Benadryl will numb the pain and the Maalox will coat your mouth.
There is no cure for herpes, but your doctor may give you a prescription that works to reduce the incidence. Some people notice that when they don’t get enough vitamin C they have more outbreaks. Foods rich in vitamin C are citrus fruits, tomatoes, potatoes, Brussels sprouts, cauliflower, broccoli, strawberries, cabbage, and spinach. You don’t need an excess amount, just 75 milligrams per day which is the RDA for women 19 to 70+ years.