Hello everyone,
Thank you so much for all of your responses! I really appreciate it. I talked to my mother again today and while she's still adamantly against a service dog, I've decided to go ahead with the process and apply for a service dog with Guardian Angels Medical Service Dogs, Inc., without her blessing. I've realized that she doesn't completely understand my situation and what I'm going through right now, and that she thinks that I'm just a stupid, irresponsible teenager, saying "you don't see what I see." But she doesn't see what I go through, having seizures and paralytic episodes with a worrying frequency, dissociating, having anxiety attacks and flashbacks, all my sleep problems, becoming more and more withdrawn... basically, she doesn't understand that I have PTSD. She thinks I have a mild form of GAD and depression. I don't know how to explain to her that every day is a struggle for me, and that even basic tasks take triple the energy that it takes her.
In any case, I'm currently filling out my preliminary application for a service dog! Yay!