Write any letter you want and put it on here as as outlet but don't send it or give it to her. That would be full of you pursuit and more of the same to her. Here's the thing. She knows you love her,she knows you want to do anything so save the M, everything you said, she knows you will say that. She doesn't want/doesn't believe it. Everytime you say "things will be different" you are showing her they wont. You want to save your M, stop talking.
Consistent actions over time are what will show her. Not just a few weeks but months and months. When you change for yourself, that is the only time she can believe you will.
Here's an example. You said before she was angry that you put yourself first and didn't consider her. Right now she wants to be single and separate. You want to be M. You not accepting her feeling and still putting yourself first,that is showing you haven't changed.
Move forward with your life and change, for you. It may just be what saves your M.
Accept what is, let go of what was, and have faith in what will be