Oh, this is so common, I think a lot of us here understand this. I barely spoke to anyone about my mum. I only told my aunt about a year or so ago. People 'knew', but not the extent of it – or how it was really affecting our family.
It's so hard to carry. I started posting on here about a year ago, and I read some books too. It always surprises me how similar the stories are, even down to small details. Learning from others didn't change the situation, but I felt a lot less alone. It transformed it from some weird, odd, shameful thing, to a sad situation that many people understand. I remember our kind neighbours told my dad, when they realised, that they knew several people living the same life.
Is there really no one you can tell in RL? No, kind, gentle discreet person? You wouldn't be letting her down. When someone is an alcoholic, everything revolves around them, and the people closest to them get so small in the face of the addiction. You deserve to be listened to and supported. You've done nothing wrong and it's not your shame.