How did you end up with your step-dad's last name in the first place?
I really have nothing of value to add, other than from what you've described, I would change your last name back to your father's last name.
By your description, I have what I think is a pretty good guess to what your first name is. πIn my experience, people who don't like their first names either alter it somewhat (Jonathan / Jon / Jonny), or, what would probably be a good option for you, is to go by your middle name. I knew a few people growing up that went by their middle name rather than their first name.