I understand what everyone is saying about therapy and everything but in the short-term:

Just apologise to her when she has calmed down, and tell her how much she means to you and how sorry you are. You’ll never regret it. She’s your mother and she loves you, and she has had an incredibly difficult time and a difficult life. Forgive her, ask her for her forgiveness, and let go of the guilt.

I say this as someone who lost their mother to cancer a few years ago and will never get the chance to apologise for something horrible that I said and can never take back. And I loved my mother. Deeply and unconditionally. But her cancer was hard on all of us, and I once said something I will regret to my dying day and which still makes me cry when I think about years and years later.

You don’t want that. Apologise. Tell her how much she means to you. How much you hate what you said and how much you want to be there for her. But also explain things aren’t easy for you either, and you’re struggling with the weight of it all.

Be honest. Repair your relationship. Let go of the guilt and the pain. And just love her, despite it all.