How do I convince them I'm ready to leave?

I’m seventeen, turning eighteen in less than seven months and I’m ready to escape the hell hole I’m living in. I live with my mother and her boyfriend and I just do not get along with them. It’s not because I’m the “rebellious” teenager. It’s because I use to be really close to my mother, that was until she started dating her now current boyfriend of four years. When his son came into the picture about four months after they had started dating my mom kind of abandoned me. She spent all her time with them and then my eighth grade year (I’m a junior in high school now) we ended up moving in with her boyfriend and since then things haven’t been the best. My mom and I are almost always fighting and when we aren’t is when I’ve locked myself up in my room away from everyone and I can’t live like this anymore. I’m tired of being ignored but when I mentioned moving out, and living with my dad my mom got very mad and threatened me. She even grounded me to my room. She gets furious when I mention living with my dad and step mom but honestly I see no reason for her to. She pays no attention to me anyhow and treats me like a child. Her and her boyfriend do.
My boyfriend and I have been taking a lot lately. About moving in with each other either on our own or into my dad and step moms house. They are completely fine with this but I have a feeling my mom will go crazy if I tell her I’m moving out, to my fathers house with my boyfriend. My boyfriend and I have been together for more than a year now. We have our arguments but we always forgive each other for what ever it may be. We are supportive of one another, always there through bad times even if we don’t feel “up” to it or like we can. We are even thinking about getting married after high school but it’s not certain yet. My mom and her boyfriend don’t really like my boyfriend, that being because he’s not the best student or the greatest person around (in their minds because to me he’s absolutely amazing and I couldn’t ask for a better man) but is any person? I know I’m certainly not the best student but I’m also not in the best atmosphere for me. I’m not happy where I’m living.
How do I let my mom down easy when it comes to me moving out in the summer. I turn eighteen in May and I plan to hopefully move out soon after that. Do I tell my mom I’m leaving and just leave or do I try and talk to her about it? Do I just leave and write her a note? If so I’m going to have to talk to her at some point unless I just never see her again.. but I plan on having children and I don’t want to keep them from their grandmother. I’m so lost and confused. Please help.
Thanks for taking the time to read this and thanks if you take the time to reply and try to help me.