HOW CAN GIRLS BE SO MEAN?

how come the girls who can be super nice are the same girls that can be the meanest chicks at the same time. Why are girls hot one day and then cold the next day with no reason. How come when a girl has a problem or stress, she automatically makes it her boyfriends problem and stress. Like when my ex GF would get mad about something someone else did or something that happened she would take her anger out on me, literally being a complete b**** to me when i didnt do anything. how come girls think that whatever a guy posesses in materials or money, that she deserves a certain percentage. Like my ex didnt drive and she lived a half hour away, and she would expect me to drive to pick her up whenever she wanted. I have a little money saved up, and when she would see that she would think that i could just buy her stuff and it doesnt matter because "you have all that money, giving me this much money would be like nothing to you" why are girls like this, why do they take advantage of a guy who has a hard time being mean to others. it really depresses me that because of who i am that ill never find a girl who wont take advantage of my generosity, i hate being nice, i wish i was naturally mean.