My mother is a jerk...?

Usually when things like this happen it means that shes trying to say something. Maybe she's going through something her self or she could be trying to live out what she wanted in her younger years through you. She knows your smart, she calls you fat because she probably wished she had as much confidence as you growing up. She tells you your going to be a failure because she most likely failed something in her life. Just tell her shes hurting your feelings and has been for years, whether it LOOKS like its effected her it really has. She won't let you see but its there. Don't hold back on your thoughts let them all out and put them all on the table, to put it simply smack her in the face with words. And make her feel like she needs to change or she'll regret it later by you either leaving or ignoring her completely.