I don't have experience with such a situation but a friend did. She'd keep on sort of splitting up and getting back together with her boyfriend and always be second-guessing herself. But one day she really just felt ready and cut things off with him. I think the reason it worked that time and not the others was because she really chose to end it--she stopped talking to him and giving him ways to get back into her life. I think with a guy like this, you really need to just cut things off cleanly and give him no inroads. My friend literally stopped taking his calls, ignored his emails, blocked him on social media, etc.
I do have some manipulative, emotional people in my life and have gradually started concluding there's no way of "controlling" these people, their behavior or their effect on you. Some of them you just don't have a choice for a while, like maybe one's parents or coworkers. But in this case, it sounds like you're really going to be better off alone. Remember, the more time you spend on this guy, the less time you have to find a new partner--someone who will probably treat you a lot better and make you a lot happier. You've probably learned a lot from this difficult relationship that will make your next one better.
Regarding the therapist, could you send her an email explaining the situation? Sounds like you're already scared of your boyfriend's outbursts no matter what. Honestly, with a guy that volatile, I'd be tempted to break up with him over the phone and then just hang up when he starts yelling. Is his temper so scary you need to involve the police?
If you're unable to commit to a decision, maybe you could try writing your thoughts in a journal--list all the possible things you might do and how you see them working out. It might help you attain some clarity through all your strong emotions.