(Sorry in advance - wall of text incoming!)
Whenever you think of backing down and going back to him, just imagine how much worse the future will be. Imagine what a bastard he'll be about this situation, how he will use this against you for as long as you're together! Remember what a bastard he is about everything else and how unhappy you've been throughout the relationship. Remember that abusers never stop abusing, and that it always gets worse. And come back here for support if you start thinking of it! We will be here.
Abusers never get better. They ALWAYS get worse.
And remember that these thoughts you're having are HIS and not yours! They are him conditioning you to feel sympathy for him, to pity him, to cave in to his demands lest you upset him even worse.
You are absolutely fine without him - much better off, even. You can and will cope, brilliantly I'm sure. You do not deserve his nastiness. You are not to blame for his stress. You gave him plenty of understanding and sympathy and it's never been enough. You deserve happiness, not this pain. And you DO NOT have to stay in this relationship!
HE chose to be a monster and mistreat you. HE chose to take out his "stress" on you. HE chose to cross the line again and again. He chose to take you for granted, to push you around, to be horrible to you. He is not your responsibility. He is a grown man who can cope on his own. He does not deserve a punch bag that he can mistreat. He does not deserve you.
Personally, I don't think you should even contact him tomorrow or answer when he rings, at all. I know it will be tempting, especially if he is relentless and driving you mad, or if he starts texting sweet sounding lies like how sorry he is and how he'll change.
Relentlessness is a tactic that abusers use to grind you down into simply giving in; they know that, in the moment, it's much easier to give in and shut them up than it is to deal with their incessant annoyance. And telling those lies (very convincingly) is another terribly common tactic abusers use; there is usually a "honeymoon" period after a massive row, in which they act unusually sweet and lovely, but then it's straight back to the abuse.
But remember that it's only temporary annoyance, instead of a lifetime of pain and abuse.
If you do communicate with him at all, let it only be to tell him that it's over, to stop contacting you, that you are serious, that if he does not stop contacting you you will report him to the police for harassment. If you do contact him and have a conversation with him, that is basically telling him that you are not serious about ending it, that there is a chance for your relationship if only he keeps ringing. Every time you acknowledge him, it's encouragement that he should continue to wear at you until he grinds you down into compliance.
There is literally NO REASON that you should subject yourself to a relationship with this man. You're not married to him, you don't have children with him, you have your own place and independence, and splitting with him is as simple as saying "it's over". You get NO enjoyment from a relationship with him; you get nothing but fear, intimidation, and abuse. Remember that. And remember that there are millions of good men out there who would be lucky to have a lovely woman like you, who would realise that and who would appreciate you, rather than abuse you. He is in the minority of monsters, and most men are lovely blokes who love and respect women and treat us well.
So let him know straight away that you are finished, for good and all, and that he must accept that and move on.