I need help getting a person with dementia to take a shower. Please?

I am taking care of my aunt who has dementia. I am 18, and this is the first time dealing with this kind of situation. She has given us a very hard time to take a shower. Now she got diarrhea everywhere, which i just cleaned up, but its all over her pants and she NEEDS one now. I am trying so hard, I feel like crying cause she wont take one. Ive tried everything, to telling her how a bath will make her feel clean and shell feel better, I gave her time to relax, I told her if she takes one we can go out to lunch. Nothing. I seriously need help right now.