I relate to this a lot. I'm also someone who goes through life carrying around a lot of anger, even if I try really hard not to let it affect other people, so I know exactly what kind of "demon" you're talking about.
The only advice I can really give is what's helped me the most. Whenever I catch myself having a negative thought, I don't try to push it away or act like it isn't there. I just question it. I ask myself stuff like, why am I feeling like this?Why did I react that way?Is this actually helping me or is it just making me feel worse?What am I even getting out of holding onto this? A lot of the time I realize my brain is just running on autopilot, repeating the same patterns over and over even though they aren't helping me anymore. It doesn't magically make the feelings go away, but it does make them feel a little less in control.
And about what you said, about feeling like you're "cosplaying" someone with passion… I honestly think that's just your brain being cruel to you. If art, volunteering, or whatever you're doing genuinely meant nothing to you, you wouldn't keep coming back to it. The fact that you want to care, that you keep trying even when you feel disconnected, tells me there's still a part of you that does. Anger, burnout, and stress have a way of making every genuine emotion feel fake, but that doesn't mean it's true.
I know that's way easier said than done, but don't let those thoughts convince you to stop doing the things that give you even the tiniest spark. I don't think those things are proof that you're pretending to care; if anything, I think they're proof that some part of you still wants to connect with the world. Even if that part feels really small right now, it's still there.
I really hope things get a little easier for you soon. ❤️