Once you know how to use your fear as a weapon, you need to build your fighting mindset.  This is called many things: killer instinct and controlled insanity are a few. But the best definition of this is a want to destroy.  You have to want to destroy your opponent, though you do not have to do so.  Understand, you do not have to beat them to a bloody pulp, you can still just hit them and get away.  This mindset just makes you want to do so, and in doing that, it keeps you from holding back.

A good drill to develope this is: stand perfectly still with your head clear, and at the sound of something (a noise, beep, start of a song) start running foreward while punching with as much force as possible and as fast as possible.

The drill looks odd to say the least, but it works. 

Another good point about this mindset is:  you can think of the situations that would make you want to do this to someone. Thus, answering the question: What would make you want to fight someone?