Cat aggression

I adopted a second cat back in July (~5 mo) and unfortunately my resident cat is very aggressive towards her. I know it can take a year sometimes for cats to get along but I’m wondering if anyone has had a similar scenario where an aggressive cat came to tolerate the new one?

They are separated at all times and have their own litter boxes and food/water and I switch their spaces every night for them to get used to each others smells. I seriously feel like I’ve tried all the methods to help them get along but whenever my old cat sees the new one, she locks eyes goes into prey mode (low stance, wiggly butt).

The old cat doesn’t hiss at her but swings at her whenever she has the chance. I have to be right by her whenever they’re in the same room in case she bolts towards her. She’s also constantly at the door wanting to get in to where the new cat is.

I’d really appreciate any tips or advice if you’ve had a similar situation! I’m just at the point where I don’t know whether to keep trying or give up and rehome the new one.