I'm really about to bring her back to the shelter.

I've had my cat for a little over 3 months now and I absolutely love her but she's seriously such a brat. I've tried everything to get her to trust me. I've gotten her so many toys and she doesn't even play with them. She's miserable and no matter how hard I try to make her feel comfortable she just lashes at me. There's times where she'll let me pet her and she'll be all sweet but then if I stop and go back to her a few hours later she'll attack me. She's even attacked my boyfriend and she loves him. But she got him so bad that she ripped the palm of his hand open and bite his thumb so hard that he needed stitches. I adopted her because they were planning on putting her to sleep the next day but she's only only a year old so I wanted to give her a chance at having a good life. I know some cats have a skiddish personality I've had cats like that before but this is out of control. She's always scared and she pees and poops where ever she is when she gets scared and she always attacks when you try to play with her. And no its not a playful attack it's a full blown attack like I'm harming her or something. Idk what to do. I really don't want to bring her back to the shelter but she's just out of control. I thought adopting a cat would be awesome but all me and my boyfriend fight about is the cat and how he doesn't want her because she's so aggressive and it'll break my heart if I do bring her back but I don't want to force her to live on our home if she isn't happy.