Just focus on concrete steps you can to leave. If you're living with him, figure out where you want to move out to and then do it.
If you think he's going to try to manipulate you into staying, then have things already lined up before you tell him what you're doing. Don't give him any room to argue with you about the matter. Simply say you are definitely leaving and then end it at that. Don't even tell him why you're going as he'll just turn it around on you. Then when you're gone cut off all contact completely. Block him on social media sites, don't pick up the phone when he calls, don't let him in the house, etc. Just stay away.
If you're concerned you'll go back due to missing him, write down a list of all the reasons why it would be a bad idea so you can look at it when you start to waver. Also, keep reminding yourself that the pain is only temporary. The sooner you break-up and the less contact you have with him, the sooner you'll get through this.
But just try to take it one step at a time. Focus on doing what needs to be done right then. Don't let the enormity of it all overwhelm you. Good luck!
“This planet has - or rather had - a problem, which was this: most of the people living on it were unhappy for pretty much of the time. Many solutions were suggested for this problem, but most of these were largely concerned with the movement of small green pieces of paper, which was odd because on the whole it wasn't the small green pieces of paper that were unhappy.” ― Douglas Adams, The Hitchhiker's Guide to the Galaxy