Gosh, this sounds so familiar...I felt this way a few times with a few of our cats too. The intro phase is the hardest part of getting a new pet. By far. ((hugs))

I would give it a little bit more time. I don't recall what you've tried yet, but do you have any of the Feliway plugins? Have you tried Rescue Remedy?

Lots of cats, especially female cats, don't like new pets in the house and it takes a lot longer for them to adjust. Something like this always disrupts the happy balance and over time it will get better.

Lots of hugs and good wishes,
Kelly