My cat is selfish? what do i do?

I have only owned one cat my whole life and she just turned two and is the only animal in the house. I love her to death and sometimes I spoil her ..but who dosent? Well I've tried a few times to bring home a new "family member" so she has a playmate and someone for company. But every single time I try...she hisses and tries to hide or attack the new animal...whether its a small dog or a little kitten. I really want to open up my home to another pet...but she's so selfish about my attention. I want the other pet to be safe if I leave them alone. What do I do?