I know she uses the word resentment but before you used it that is the word that came immediately to my mind. She sounds extremely resentful and she is the only person who knows why this is so strong. She sounds like she has been resentful for a very long time regardless of what she said. She even uses the words herself.
She gives elaborate examples of her resentment and mentions other people that have contributed to this. Situations seems to have made her resentful and you need to ask her why this has festered for such a long time. If she is willing to go into this in written form then she should be able to discuss this rationally in person. No one writes such a intense letter unless they want to get this off their chest. If they hide these things they aren't ready. You now should approach her about trying to figure this out. There may be pain there but you have to start somewhere. Be ready for anger but don't anticipate it yet. Think about the situations she describes. Think about how she may feel about them.
You have an obligation to figure this out if you want a better relationship. It's okay for things to be emotional as long as it's for a better cause. Find a time to meet with her alone. Tell her you want to talk about this message. Then she is able to make her own choice. Give her that choice. Be prepared for this and anything she feels. If you think it would be better to do this in a family therapeutic setting that is fine too. Whatever is wrong you have to embrace because she has a right to her own feelings.
If this has been helpful press accept
I was looking for a different answer, as I had already surmised the answer you provided. I need more pychological analysis. She is resentful yes, but did you not pick up on anything in the letter that could be causing this? The way the letter is worded and so on?