OP, I want to give you a suggestion that I don't think has been made yet. I have a mother a bit like yours, and I know that i will never cut her off, despite 'things' she's my mum, and when she's nice things are fine. But over the years she has been absolutely toxic. She has made the hardest times in my life worse, every single time, by orchestrating a big row with me at the worst possible times. (Like the night before I went into a c-section where she was due to be birth partner, for example).
I'm telling you this because my advice would only work if your situation is similar to mine. I have got wise to her now and do not let her manipulate me, and on the whole, things are good. I do not, however, forget. that is important, I think, as she can blindside me now and again if I get complacent.
What I would do is this: Put the letter away. be honest with her about the reason. tell her you're not in a good place right now and you're not going to read it yet, but one day you will. Mention it in passing every now and then. How soon, you're going to climb into the loft and get it back. So that you can read it when you're feeling stronger. Now, if she has any conscience at all, it will eat away at her. Knowing that it's there, and that you're going to get to read it.
If you read it, and react, then your reaction will become 'part' of the row. If you bin it, and send it back, then you will never know what was in it, and she will lie. If you do my suggestion, then you have all the power. Not only do you decline to read it now, when you don't feel up to it, you also have her words as proof of what she is like. And she has this hanging over her. She cannot decide to move forward at any point. Any attempt to be nice to you will always be overshadowed by the fact that you have the letter. She can't apologise or explain or make excuses, because you haven't actually read it. She will be the one waiting, waiting, waiting for the dreaded phonecall, not you. She will be the one lying awake at night wondering if you'll read it today.
After all that, if you're a total bitch, like I am you could invite her round for dinner one Sunday and bring it out with the apple crumble! 