are you certain it is a cold sore?
They are caused by a virus and I don't think you would just get one out of the blue (no expert on that). Stress is a common cause of them resurfacing
Alas the best thing to do it let the air at it and give it time to heal - be really careful of any "ooze" that comes out because that can be infectious.
If its any help, it probably feels way worse to you than the folks seeing it