I know exactly how you feel. I was stressing about the same thing. Sage was used to being an only cat. She was our first "no cage" pet and was over a year when we decided to bring Ska home. But I was afraid she would feel like we loved her less. I was afraid I'd love her more than the new kitten. I was afraid I wouldn't be able to divide my attention equally between the 2 and Sage would get jealous. We brought Ska home and she HATED him. She would try to bite him and she was so mad she'd bite us, she drew blood on Dave once. We almost gave Ska back, we thought there was no way it would work.
Now, they are fine. Ska annoys Sage since he's still a kitten and pounces on her when she doesn't want him to. But they do play, they chase each other around the house and I'm happy we got another cat to keep her company.
It can work, but you just have to not give up. We were ready to give up within a week, but then slowly, they would sit on the same chair together, no hissing. And slowly Sage would walk by him, not pouncing.