Why is my dad so mean?

Nothing I ever do pleases him. He's always ordering me around, and making things really unfair. Like for example he was on the phone talking to my mum and I asked to talk to her after him and he ignored me and gave the phone to my brother, and because I have a problem around people eating he picked up a peice of gristle from his plate and started crunching it really loud. I just know he did it to make me go away. And just earlier I was eating sweets my cousin had bought round for my mums party and he went "No wonder you've got spots Ellie" really nastily. He's sworn at me before, and whenever he says something and I haven't heard it and I ask him to repeat it he just looks ast me like I'm nothing. I get really frusrated, and some things he says really hurt me. But sometimes he'll be nice and I'll forgive him and he will just be horrible again. He's so nice to my brother, but not to me. And I do get angry with him and shout at him when he orders me about like a slave, but I'm only 13 and I just want him to be nice all the time and not just when my mum yells at him for being nasty or he feels guilty. Please help. P.s. the problem around food orginated from him being such a disgusting eater, and I'm very anxious all the time due to the fact he'd verbally abuse my mum really badly when they split up when I was little. And he's got schizophrenia. Please help? :'(