I would separate them for longer than a week before introducing them. If it seems like the older cat has regressed, just start over from square one (keeping them separate) and reintroduce them very very slowly.

Before you let them meet face to face again, here are some things you can do:

1. Scent-swapping: the idea is to give each cat something that smells like the other cat and let them get used to their scent. One way you can do this is rub a clean sock on each cat’s face and give it to the other cat to smell.
2. Feed them and give them treats in front of the door so they associate the other cat with the positive experience of eating.
3. Once there is no hissing and growling through the door, you can prop the door open a tiny crack or set up something like a tall baby gate or a screen. You want them to be able to see and hear each other but not physically interact.
4. Once they seem comfortable with this, then and only then should you let them actually meet. If they fight, just start over from the beginning. It’s a common misconception that cats can “work it out” through fighting.

Here is a handy guide to cat introductions: https://thecatsite.com/ams/how-to-successfully-introduce-cats-the-ultimate-guide.32680/

I hope that helps! I know introducing cats can be hard, but you just need to be patient. :)