Visualization, visualization, visualization.
It's crossing that threshold into the unknown is whats holding you back. It's not the fear of fighting but the fear that your entire body/mind has about entering into a situation in that specific kind of way that it is unfamiliar with.
You don't turn your fear into anything, think of it as a pathway into your accessing your emotions. The fear is going to be[Full stop] It's the way your mind/body perceives the situation is what dictates how much your body is going to pressure you mind into making a decision.
Fear is the foundation of how you will make your decisions, fear doesn't turn into aggression - it supports it. Your mind is going to make a decision to fight or not to fight.
Sounds like you have never stepped into that aggressive situation before and every-time you made that choice - no bodily injury - things turned out pretty good in terms of avoiding injury = Body is happy!
Stepping into that situation you have to have already made the decision to fight, or more so that it is an option you are willing to take. You have to visualize in your mind in your down time or just day to day life that it's a choice that you have made and act upon and now your body is going to be part of the consequences.
If your body feels like the mind has taken control and the fight is going to happen, your body announces "ante up, lets do this!!". The muscles and reflexes that you have effectively honed will start to fire off like crazy, your will get that adrenaline dump and your body will send the signal to your brain too destroy what ever out there is causing the stress internally.
tldr; You have to have the mindset before hand that fighting is a situation that your have already made the decision to enter and your body has to deal with the consequences. It's getting the body to realize "Fuck it, were doing this live! Activate fight mode, EXTERMINATE!!!!"
Start with visualization, there are so many things that are involved so it's not an easy process to master overnight.
If I had one wish that could be granted, it would be to face/know my greatest fear. So seeking out and confronting one of yours is something I can respect.