Take anger manager classes. There is nothing you can do, except not plead guilty and hope a jury sees it your way. You grabbing the phone is considered and assault, and you admit to doing that.
They have every right to ban you from the store as it is private property and they elect not to have you on it..
And if somebody asks you how you are doing and you go all ape sh*it on em, you do have some issues. She was being customer friendly by simply asking how you are doing, and you go off on her, you grab the phone out of her hands, you are lucky you were only charged with what you were.
You were annoyed with the store, so what? They have recourses for you to express yourself, and by grabbing her phone out of her hand is not one of them.
You are banned and no court will over turn that, and you must also remember that your temper tantrum was probably recorded on the stores surveillance system which will be shown to the court/judge if it goes that far.
So now you will have a criminal record as well if convicted (which you probably will be), you will have lawyer fees either way, and if you are guilty you will probably be billed for Wal-Marts attorneys as well.