My heart is aching for you, KMPrim.
I have a child who has gone through something similar -- gradually losing respect for the hero of his childhood because of alcohol. I've seen how incredibly painful it can be, and I've also seen how you as a young person can come to take on way more responsibility than you should have to. I was that mom, like your mom, whose son told me when I left his father, "I wish you had done this years ago."
I will tell you the same thing I've told my son: None of this is your fault. He won't drink more (or less) because your grades go up or down, you behave better or worse, or you act out or are obedient. You have to do all the right things for your own sake because you have no control over your father or his choices.
You also don't have any responsibility to "help out" your mother in all of this. What you do have the right, and the responsibility, to do, is help yourself. My son felt really stuck, not being able to trust his school counselor and feeling too ashamed of what was going on at home to want to talk to his friends. If you do have someone you trust -- a counselor, teacher, pastor, coach, talk to them. Because you also don't have a responsibility to protect your father. You need to get whatever help you can for yourself and leave the adults in your life to sort out their problems.
And please do check out the ACOA part of the forums. Because nobody can understand what you're going through like other people who have been there.
Lots of love and hugs to you. I can tell you're hurting, a lot, and it breaks my heart.