If it helps you to think of it a little differently, can you try and view it this way:
If you can't do anything right, why is he still there? Because he needs someone to do all the grunt work in his life for him, and because he enjoys having power over someone
If he's afraid of what may happen next time, why is he still there? Because he isn't afraid at all - he isn't concerned about you at all. He is warning you that he can do more damage
He says he wasn't like this with his ex. I'm fiery So he instinctively beats fiery people who question him in to submission? Of course he doesn't, because f he did he'd be in prison.
You know he does this because he can, and because it gets him what he wants.
You know you need to leave so telling you that isn't helpful. But please step back and think - in 20 years time, do you want ds's girlfriend terrified of him, because he thinks beating her up is normal?
Try and imagine something nice but ordinary - Christmas Day or Ds's birthday - imagine getting up and having breakfast, giving him some presents, eating what you like, going where you like, joking and having a great day. Imagine no eggshells to walk on, no dsd coming in and taking over, no rows, no shouting. Just you and ds having a giggle together. Your life could be great OP, if you can get through the hard part, you could have that peaceful life that you deserve 