I suspect you're using the term cold sore incorrectly.

Cold sores are the herpes virus and typically show up on the outer lip.

I suspect what you have is recurring mouth ulcers which show up inside the mouth (these are not the herpes virus).

Chronic, recurring mouth ulcers are typically caused by two things

1 - Improper nutrition, or
2 - GI chronic condition that affects absorbtion of vitamins and minerals.

Bottom line you most likely need to eat more vegetables and fruit. If I were you I would see a doctor about it, though.

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Michael

1988 300 SL (5 Speed)
1994 E320 Wagon
1997 C230