It sounds like the inner demons that you’re battling are caused largely by your mother. You’ve internalized her voice (the one that calls you fat, untalented, and dumb) and that’s created a lot of needless pain for you. Your mom is insecure and gets a dose of self-worth when her daughters excel. When you don’t reflect well on her, she feels hostile toward you and calls you names.
I don’t know your situation and options so I can’t recommend ways for you to escape. If you feel that it’s intolerable to stay at home, ask an extended family member or a friend’s family if you can move in with them. If that’s not necessary, ask someone to speak with your mother on your behalf. That person (your dad, your sister, an aunt, a grandparent, a neighbor) should explain how destructive the name-calling is. They should encourage your mom to see a therapist or take parenting classes.
One of my favorite mantras is: “Comparison is the thief of joy.” Don’t let your mom or anyone else measure you against your sisters and find you lacking (that includes yourself)! You’re a unique and talented being who’s here to do wonderful things. Don’t use the excuse of having inner demons to keep you from enjoying life, helping others, and accomplishing your goals. If you need help to manage your thoughts, work with a cognitive therapist online or in person.