I'm so sorry you're going through this . Unfortunately I don't have any experience in introducing cats together, you might want to give it some more time though, hopefully Bella will calm down soon.
My cat happens to be very territorial and when I adopted him I was warned about it and have never attempted to bring another cat in. And considering the way he reacts when another cat is hanging around outside I'm glad they let me know.
So I'd just give it more time and be thankful that Bella isn't trying to kill poor Summer like my Mel would