This can be a really hard situation, and there are lots of different ways to go about this kind of thing! I think that above all, just keep praying for your mother to have a change of heart. It happened in the Bible, and it happened in the Book of Mormon, and it happens every day for our missionaries and members alike who are struggling with someone. Pray that her heart will be softened, and stay close to the Spirit so that you can be prompted what to say and what to talk to her about. Also, if she thinks that seminary would complicate your life, find out if you could have someone pick you up and take you to seminary each morning so that your mom doesn't need to worry about it.