:hug:sending you a warm welcome, so sorry you lost your friend like that it must have been such a shock and close friends like that not only rare but often closer especially through our teenage years than familly.Anger is a normal part of grief so what you are describing isnt something you are alone with. Sometimes counting to ten just isnt enough though is it.
It will pass, but it needs to be expressed. More gentle ways can be in writing or art and by expressing it, -the injustice, the unfairness, your feelings of utter loss that are fueling it, you create less chance of the hot rage outbursts happening.
When you feel one building though different things work for different people.
There is a hill behind my house and the way i dealt with it was to walk up it as fast as (age and infirmity allowing) i can -By the time i get too the top -or before that i,d worked it out of my system if not then the view from the top helped to be calming too. Not everyone is lucky enough to have a conveniently placed hill but if you can find a park or a safe space, then brisk walking or jogging might work and a pair of earphones will put off the interuptions and block distractions, only once again -please make sure its a safe space to be doing that.
Others here have driven out to remote places and screamed -thats not for me but i think it helped them. Also a bottle bank and a stash too deposit in it -if you have one nearby -that way you can throw bottles without comment from or harm to anyone.