First, the em-dash has to go. You could write the sentence as, “In my world, I was a nerd, but here I was king” and it would mean the same thing. That shows there’s a clear connection between the two parts, not an abrupt change of direction.
I opted not to add an extra word, though, and break the example into two sentences instead.
“In my world, I was a nerd. Here I was king.”
Is this character no longer king wherever here is? If he’s still king, that sentence should be changed to Here I am king. (Being a nerd and being a king are not mutually exclusive, by the way. *mutters something about computers and chaos*)