I live in Sydney, and I have to wear a mask at work and when I am out shopping, inside the shopping centre. I also have to wear them on public transport, but I don't use public transport anyway.
I was absolutely fuming when I found out that they mandated masks, and in the early days of enforcement. Since then, I have grown used to wearing them. I understand, though, that having to wear them for twelve hours is a very uncomfortable proposition. I was really struggling with not being able to see people's faces, but that was more of a symbolic thing, not because I actually have anyone to see. You can read my thoughts about it on this post.
I'll be honest, at work when there is no-one around me, I put the nose down, and that is way more comfortable. I know that's not what you're supposed to do, but I'll be a bit naughty in that regard. You can also find other ways to get relief around the rule. For example, you don't have to wear a mask while you're eating and drinking, so bring a water bottle or snacks with you, and whenever you need a breath, stop for a second, try to get some privacy and have a drink.
You might want to look up the exemptions to the rule, because I know some forms of mental illness qualify. That could seriously help. If someone gives you grief for not wearing a mask, remind them that it is the role of the Police to enforce the rule, not citizens themselves. Considering the situation is having such a paralysing effect on your mental health, I don't think you would have trouble getting an exemption if it's within your rights. You may also be able to get an exemption if you claim that your job makes wearing a mask a heat exhaustion risk. I know it gets REALLY hot behind a coffee machine.
Finally, consider trying different types of masks, with different materials. This may also make you feel more confident in them, because you can make it a way to express yourself. I have found that the fabric ones look really good, but they are so heavy that they get a little uncomfortable. The blue surgical ones feel and work better but look really clinical, no pun intended.
That's all the specific advice I have, but in a general vein, please don't worry too much about the announcements of the virus. Queensland is REALLY aggressive about their management of the virus.. like I swear, they will probably get one case and go into a Stage Nine lockdown. The case numbers are beyond your control. You're doing your bit. Even if you get the virus, which is highly unlikely, there's a process for that.
You're going to be OK.
All the best x.