Whenever I hold my snake it poops and pees on me. Is that just because she doesn't want to be held? Will she ever stop? Is there something I can do to make her stop?

That is a defensive mechanism. It feels threatened. All you need to do is handle it more frequently and make sure you're very gentile with the snake and make it feel comfortable. It will still poop and pee on you, but this will happen less with time until it doesn't see you as a threat anymore. Good luck!