@maddison You are going through exactly what I went through with my sibling, I feel from reading your posts that I'm right back there in the worst of times.
What is your situation like exactly? Can you separate from this person? Can you make him leave? Can you leave? What is it that ties you together at the moment?
As everyone will tell you, you don't deserve any of this and you need to stay strong and avoid any unnecessary contact with this person. I know that's hard when you live together but there are some things you can try.
The main thing is to not occupy the same space at the same time.
If he is in a particular part of the house, you get out of that space. Doesn't matter at the moment if you were there first, just go and do it without saying a single word. If you are doing something you can't leave, such as cooking, just go about your business and don't say anything to him.
Keep your communication to a bare minimum - by that I mean what you say to eachother. Don't talk to this person unless you absolutely have to.
Say no to anything you don't like and mean it. If your no is not heard, you can repeat it but leave it at that. You do not need to justify in any way the fact that you said no. Sometimes people with a mental illness will bombard you with demands or wants. You may need to say something like "let me think about that for half an hour and I'll get back to you". Then do exactly that. Don't leave them hanging either because they will be watching the clock for when that half hour is up.
If this person wants to engage you in an argument or anything negative just say "sorry but if you are going to yell at me, abuse me etc etc" I'm not prepared to listen to you". "When you calm down you can talk to me". Then remove yourself from the situation immediately. If he wants to keep ranting in an empty room let him. That's his problem then, not yours.
Keep any conversations clear and to the point. Don't let him ramble on and on. Ask him to get to the point. If he doesn't, just walk away.
Always, always, always mean what you say. The way someone with a mental illness thinks is vastly different to the way we think. They will take everything you say as gospel so be really careful. Think first about anything you want to say to him. Try not to get into a slugging match with him where you might say something you regret. You will forget what you said but he won't and will use that against you at every opportunity. This is another reason why I suggest zipping it for a while. If you keep the communication to a bare minimum he can't twist stuff you say around to hurt you. For some reason they seem to feed off our misery.
I also really want to encourage you to keep using the help of police and the cat team when you need to. Remember, that is what they are there for. There is not enough help for the mentally ill in our community and I know the police are overloaded with this stuff but the system won't change unless things get so bad for our friends in blue that the government offers more assistance. Do not ever feel that you shouldn't use their assistance.
You absolutely must seek some counselling for yourself. Go to your local gp and ask for a mental health check/report. Through the government you are entitled to a number of counselling sessions for free. Use it - asap.
Before you start drowing in this madness and your own mental health is affected, you need to start taking the power back that rightfully belongs to you as your own person.
There are a lot of good groups out there as well that I suggest you get in contact with. Remember Lifeline is only a call away if things get a bit out of hand at home. They are fantastic at what they do.
Do not think you have to take this on yourself. Big, big, big mistake if you try. You are responsible for only you.
I hope some of my tips will help you put in place a more harmonious environment. I really feel that you should look at some ways you can get away from this person permanently.
If I think of anything else that can help you I will repost. Please take care of yourself. I just hate the thought that someone else is going through what I went through. It makes me so angry and sad at the same time. Please keep posting here as well.
One last thing that I think is hugely important - whatever you do, do not isolate yourself. It can happen quicker than you think, especially when you're feeling down and depressed with what's going on. Let as many friends and family as possible know about your situation. If you work, let your colleagues know too. There is nothing worse than being alone in this miserable situation. Please make sure that doesn't happen to you.