is it unreasonable to ask for a break when you live together?

It does sound a bit unrealistic tbh esp if relying on him paying for the place and to a lot of people there's no difference between a break and a break up. Either 1 person would normally leave, esp if no children involved or it's an extremely amicable breakup..

i don't want to break up, though. i just need like a mental break from everything. is it really that much to ask?

Well, if you live together I'm not sure that you'd actually be having much of a break.
Instead of phrasing it as "I want a break", why not book yourself a solo trip away for a couple of weeks?

That's what I'm saying though, a lot of people don't see much of a difference about whether it's temporary or not. And kinda yes, a lot of people would consider their partner wanting to withdraw emotionally from their relationship and their own needs while staying there as a lot to ask.

he works a lot during the weekday so only really see him in the evenings anyway. but the plan was to - sleep in a different bedroom, stay out of each other's way, no sex etc.
he won't let me go on a solo trip, lol.

Well, he doesn't really get to dictate to you whether or not you can go on a trip. He can grumble about it, but ultimately if you book a trip and go on it he can't stop you.
I understand that you don't want to cause an argument with him, but it sounds like asking for a break is already resulting in one. I reckon being literally away from the house for a couple of weeks so you're not walking on egg shells every evening when he comes home from work is probably going to be better for your mental health if you need some space.
It would mean your partner isn't seeing it as you being on a break from him, but rather you're just on a mental health recharge trip.

i'm pretty sure he would. 'for my own good' of course.
i need to find a way to mentally 'reset' at home.

Is it depression you're struggling with primarily?

i get pretty crazy mood swings tbh. most of the time it's depression, but not right now. i don't even know what my mood is right now. i just feel all over the place. i don't know what i want anymore - i just know that my life isn't what i want it to be right now. i keep making bad life choices : /

I was going to say I've been looking at this thing for my daughter (who suffers from depression) but I haven't quite decided whether or not to get her one yet.
I wondered if it might be of any interest to you:
https://www.flowneuroscience.com/

Typically proposing ‘a break’ is a way of softening the end of a committed relationship that has run its course. It might be fairer to say you want move on and, if you can, bank some of the good times

I don't think you could/should call it a break, but you could discuss setting some boundaries or limitations on your interactions for a few weeks. Although with that will require communication.

I can’t see how you can have a break from each other living in the same home, I think it would be stressful for both of you. Perhaps one of you could go away for a week or two might be the simplest solution.
Have you contacted your GP?

it's giving electric shock therapy vibes : ( their claims seem a bit pseudoscientific but idk.
has she ever tried antidepressants?

The GP won't give her them. She's had therapy but it didn't last long and didn't particularly help.