Damn! Sorry you are going through this.
You have made the first most important step. You have realised there is a problem and you have spoken about it even if it is to a bunch of internet strangers.
Your 14 year old daughter if she has seen this for all her short life probably thinks this is the way relationships should be. Do you want her spending years living with a man exactly the same as you have?
Do you want to imagine your son, his face probably screwed up screaming the way his dad does at his girlfriend/wife because that is what he thinks a relationship is.
If he realised you have changed in some small way because you have realised he is wrong and are even the slightest bit stronger he could get much worse or go the opposite way and be sweet and loving (would you have stayed with him for over 14 years of constant abuse with out and redeeming parts?).
It is very easy for me to say leave.
It is very hard for you to do so, especially with 3 little ones who are conditioned to love their dad despite the way he is.
Please please keep posting, when he screams at you next please post here so we can all reassure you that it is HIM who has a problem not YOU.
No matter how long it takes we will be here with as much support as we can.
Please be very careful and take care of yourself.