I would definitely write everything in a letter. Read and reread it and try to remove as much tension as you can from what you tell her. You just learned you have a problem. Not only your mother, but probably everyone else who knows you has misjudged you. Provide information about AS. Tell her you know she and others had no way to know you are neurologically different from the majority of people. State that you never had a clue why you are so often at odds with the majority. Tell her how much pain all the accusations from her and other people have caused you pain. Tell her that while many Aspies try to hide their differences, it isn't always possible--at times you realize AFTER you have done something irritating and then it is too late to take it back. Remind her that this is a real neurological condition and you will always have to live with it. Ask her for help. Remind her that her criticism over the years has been painful but you forgive her for that because until she reads you letter, she will have never known about your condition. Also tell/ask her (not sure which would be a better choice for the two of you) that you hope she loves you and will try to understand you. Provide some good URLs if she uses the computer, or print some articles and give them to her to read and digest. Ask her to take some time to consider all the information and get used to the idea that you have, and have always had a real syndrome over which you have little control. Make it clear there is no magic pill that will "cure" you, that you will try to remember not to do things that irritate her, but that it won't always be possible. Tell her that now she knows about your neurological differences you will consider any thoughtless criticisms from her to be rude and hurtful.
VERY IMPORTANT! I am soon to be 70 and my parents died years before I learned, in my early 60s, that I am an Aspie. They never knew, and actually I am now certain, my father was also an Aspie. I am unable to forgive my mother for not realizing I had a problem. She was a teacher and she must have had some Aspie students who acted like me. For almost 10 years I have had this imaginary conversation with my mother. She was intelligent and if I could confront her with the facts I hope she would realize it wasn't my fault. I would definitely give her a chance to change her view and acceptance of me but if she refused, I could then walk away. I hope you find a way to educate your mother so you can begin a new and healthy relationship. My mother made my life a living hell.