The hearing voices website has a forum, and I would recommend signing on there and asking those there what types of things they have found helpful. That would be one place to start, because the peers really have been in the trenches and can give the best guidance.
Have you ever journaled your anger? It seems to me you are afraid that if you let your anger fly, something bad will happen, maybe you'll disappear or something along those lines. So, what about starting small, lettting some of it come out in a less extreme way, to start, and seeing how that feels?
I came from a protestant upbringing where we were discouraged from exhibiting uncomfortable emotions. We were never punished as such, but we felt shame for disappointing my parents. I remember as a little kid watching my parents argue loudly and feeling so distressed. As a teenage I shoved feelings down and numbed out with an eating disorder. We were supposed to be smart, get high marks, be perfect etc. so I became my own worst critic and had very low self-esteem, lack of confidence. I don't recall experiencing anger.
A type of CBT that I am doing, called Emotional Brain Training, taught me how to process emotions. You start by thinking of a situation that has you really upset. You can write this out. "The situation that I am most upset about is ..........." Then you write out your angers. "I'm angry that..." "I hate that...." "I can't stand it that......" and you do that until you can't come up with any more. Then you move to sadness: "I feel sad that...." "I'm unhappy that...." "I feel distressed that...." When you can't come up with any more, you move to fear: "I'm afraid that....." "I find it scary that....." And when you are done with those, you come to guilt: "I feel guilty that...." "I regret that...." "I wish I hadn't ......" And now, you come to your Unreasonable Expectation. You may state it as an expectation. "Of course I would feel this way about this situation, because I expect that......" Usuall that expectation is really quite unreasonable. It might be something like "of course bad things happen to me because I'm a bad person and I don't deserve good things."
Now, if anyone else were to say this expectation to you, you would see it as unreasonable, yes? So, next you come up with a reasonable expecation that your brain can accept. It might be "It's not true that I am a bad person." "It's not true that I am unworthy." "It's not true that I don't deserve love." "It's not true that my feelings don't matter."
It has to be something that your brain can accept. For me, in the beginning, it didn't feel right to say "I'm a good person," but it felt more acceptable to say "It's not true that I'm not a good person." Now, you grind it in. This is your mantra that you recite to yourself 20, 50, 100 times a day. You start out saying it very slowly, putting emphasis on different words in the sentence each time you say it. Then you start to say it faster, with more conviction. Then you say it with happiness in your voice. You can even sing it! By the end, you should feel unburdened, lighter. You repeat this every day for at least a week. You are rewiring your brain.
So, that's the long version! This is something that anyone can do for any situation that is troubling, distressing, causing anger and upset. I think if you write all this out, it will be a gentle way to ease into it, where you can see that nothing bad happens by processing your emotions. Because that is where you are stuck, right now - unable to process your emotions, and they need to be acknowledged, by you if no body else. Right now you are struggling to control these feelings, like a bottle of soda being shaken, ready to pop! You need a safe way of processing your feelings so that they don't fester and cause you to want to do something extreme.
We call these "Cycles" in EBT. You could even do a cycle about your fear of letting your anger out, that the dark presence will come.
There's also a lot of CBT self-help online, and it's just a matter of searching and finding exercises that resonate with you.