I need help sorting this out in my head :/

So, my stepmom has been married to my dad for 10 years. My birth mother is addicted to drugs and used to beat me etc so we don't talk. My father is the only parent I need in my life. This is what's clear.

Here's where it gets confusing: my stepmom is... something else... (I catch hell for calling her anything)... she doesn't understand anything. She made a joke about having to call my birth mother for "mother daughter day" and I said "I really don't like when you say stuff like that." And she said "Why don't you go fuck yourself?" And started screaming at me. I never once cussed back at her, but asked her why she's always calling me "disrespectful" when she gets to say things like that to me. My father was in the hospital near death 3 times recently. she left during the 2nd one, and didn't come back and didn't go with me when he went back again. I stayed with him every day and every night and she still screamed at me the one time I came home. She tries to control him. She's at the point now that she talks to herself. We've driven her nuts.

My dad flirts with girls a lot. Girls that are closer to my age (which doesn't bother me, my SO is 20 years older than me). And I believe that one of these women are what's really best for my dad. he needs someone who can keep up with him, and keep him feeling young and alive. and truthfully, I can't blame him for cheating (which I'm pretty sure he's doing)... they haven't had sex the whole time they've been married because she's "not interested". They barely hug or kiss. All they do is fight. So I'm confused because I support my dad in this. I mean I'd support him in anything, but shouldn't I resent him a little? Shouldn't I hate him a tiny bit for giving me bad ideals about love? I just have this deep feeling that he wasn't meant to be monogamous because he's been talking to his high school sweetheart lately and he cheated on her too. But she makes him smile. I love seeing him smile. He doesn't smile often. So am I wrong for thinking he should take off and live the rest of their lives together without a care in the world, just finishing the love they started? Maybe they could make it work this time. They're older and know better now...

And my stepmom is really getting to me. In my opinion she has no right to speak to my dad at all, and most days I don't think she has the right to live but I try to keep that down.. she abandoned him. She has no right to tell him what he can or can't do. And she's barely a wife!! All she does is complain and cuss...