I want to ask for more money after an agreement has been reached
Why do you think that's justified?
I now realize that I did not ask for enough in the first place.
You should have thought about the compensation levels in the first place. It is not okay to try to renegotiate for more money after an agreement has been reached.
I didn't know that I was getting screwed on the deal. I'm just trying to get what I want.
Due diligence in these types of deals is necessary to figure out the appropriate levels of compensation so that you don't have to go back and re-strike a deal again. Stick with the initial agreement or you will lose your integrity in the business community.