Life 101 can be a real butt kicker.
If you give up, they win. I don't know who they are, but trust me, you are not the only one they play games with. Hang in there.
Also, and I am not defending him here, but find out what your dad thinks and why he's doing what he's doing before you condemn him for it. Maybe he and your mother know it's over but haven't told you yet. Maybe he's just confused. Maybe he's just not loyal. Without an actual honest conversation, you'll never know.
And, being a girl, it might not be easy or even possible for him to talk to you about his personal problems. Some guys can't, some won't.
There are people here and no doubt at your school, maybe even a personal friend or two that would let you lean on them when you need to. It's ok to take help if you're working on whatever it is you need help with. You already know sitting on your ass solves nothing, but don't think you're all alone.
My daughter is just a year younger than you. I've seen how little some of the fathers of her female friends actually communicate. If you think it's just you and your dad, it's not.
Don't give in but don't compromise your beliefs.