To dream of a cold sore represents feelings of embarrassment that other people are think you are a permanent failure or loser. Public embarrassment, fear that other people are talking about your behind your back, or feelings about yourself being perceived by others as someone who repeatedly makes a fool of themselves. Feelings about recurring failures or embarrassments that other people witness.
Alternatively, cold sores may reflect feelings about repeatedly embarrassing yourself in front of others making bad relationship choices. Feelings about yourself looking like you are too stupid to find the right person or that a relationship is impossible for others to succeed at with you.
Example: A woman dreamed of getting a sore sore. In waking life she was having doubts about her wedding for her second marriage. The cold sore most likely reflected her feelings about all her friends and family thinking something is permanently wrong with her if she left her fiancee at the last minute after already being divorced once.