I am sorry. I thought you said that you can take a photo.
There are several common conditions that can cause a chronic sore on the roof of the mouth.
A sore can develop because of chronic acid reflux that is letting acidic contents get into the mouth. Most people with acid reflux will also have heartburn, but there are some people with acid reflux that do not have heartburn and only present with symptoms from acid irritation of the throat, mouth, or lungs.
Mouth sores can also occur because of chronic inflammatory conditions, and this would be more likely if there are other unexplained systemic symptoms.
The other major concern when there is a chronic sore in the mouth would be a cancer of the mouth. Cancer of the roof of the mouth is not as common as other types of cancer, but can occur. Any time there is a sore that persists, there should be consideration of whether it is cancer.
It is reasonable to try some home interventions, particularly for acid reflux. It may help to take a medicine to suppress stomach acid, such as ranitidine or omeprazole. It also may help to swish antacids in the mouth several times per day, including at bed time. If these interventions promote healing of the sore, then it obviously is not cancer. But if the sore persist, then it would be important to see your doctor for further evaluation.
If I can provide any additional information, please let me know.