Im afraid I may have to rehome one of my cats, help?

I'm sitting here sobbing as I type this. And I know it isn't typical content on here. But I'm looking for support where i can find it.

I am recently having aggression issues with my 2 cats starting from a stray showing up at my window and freaking one of them out. She then attacked my other cat in the midst of that. Neither were injured as I broke it up very quickly. But it was intense.
Since then, it's been about a week and they cannot be together reliably. They can be okay for a minute, or while they're eating. But inevitably one starts to stare the other down until a fight breaks out. So I've been keeping them separated unless I'm DIRECTLY supervising. Even then, that has seemed to be a mistake.
If there's a barrier between them, but they can see each other, there's no aggression. But if loose, it's definitely not easy.
I have a vet appointment in 2 days to get the initial cat that did the attack seen and maybe medicated for her anxiety (which has been an issue, just not to this degree). Then maybe see a behavior specialist, which I already cannot afford to do, but may do anyways.

I know I need to not be too dramatic before I even see the vet, but I'm so scared this is not resolvable and I am DEVASTATED at the idea of having to remove either of them. They are my world.
Has anyone else gone through something similar and had positive results?

Also, I live in a one bedroom apartment. So it's already small. Which is especially why I hate confining them to one room.

I guess I just want to get some hope from this. It's been a very sleepless and emotional week.

Im afraid I may have to rehome one of my cats, help?
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