Yea this sounds like a saddle sore. Basically just a pimple from hell. Unless it grows arms or turns colors...probably nothing to worry about other than the pain and it making riding uncomfortable.

They make stuff called chamois cream to help prevent it. I had a couple over the summer and started using my daughters diaper rash cream. Seemed to help

edit: wait...have you had multiple sores over the course of years, or ONE sore that has lasted years?!