You have had this for a very long time and it has not enlarged or caused you any problems, so I would say that is a very good sign and it's nothing to be concerned about. I was going to suggest it might be a small sebaceous cyst, but I don't think they last for YEARS.
Women should start seeing their gynecologist at least once every year when they turn 18 years old, or when they start being sexually active.
My advice is to begin seeing a gynecologist and have your mystery lump diagnosed.
I'm sure you're fine, but I understand you wanting to know what it is. I know I would.
I wish you the best
RubyWitch