How do I set boundaries without feeling extreme guilt?

I (26 F) am struggling to cut contact with my family. im not sure how to do a long story short. I left home when i was freshly 18, my mom was a severe alcoholic and i feared if i stayed in the house, id never be able to leave. she finally got sober when i was 24, around the time i started my own addiction. I knew better, but the cookie crumbled when i tried controlling every aspect of my life and when i sat alone in my house with everything i built left feeling this complete emptiness. even though i left i continued sending money where i could, to my sister and mom. the money isnt what matters, it was after she got sober that mattered, and when i fell into my own personal hell. Iv begged for conversations about what happened in my childhood, her addiction, etc. I thought this would help my trauma, my healing, because shes sober..right? Thats all i had ever wanted even as a child. now it feels like the roles are flipped. I feel left with pieces of myself, of doing for her and my sister, and i fell to the same disease. I genuinely feel like im in an extreme amount of emotional pain, the refusal of conversation, the denial, the dismissing of my emotions. she says, the past is in the past, I cant help you. I guess im asking how do I put my emotions aside to cut contact, its taking up so much of my brain space, and affecting my relationship. I have this horrible fear of if i cut contact they will pass away, and or my nephew will think as he gets older that I just left him. Is this irrational of me to feel? I almost feel like that I know the answers to my own questions, but the guilt of me picking myself is so extreme