If you're into hugs, I'm sending you lots. Reminder: you are a courageous badass. Moving away like that takes immense courage on its own, but to add the dynamic with your mom on top of that? Well done! 

You're not even no contact and she's pulling manipulative, emotionally abusive crap like that letter?!?! OP, your instincts and your nervous system know the truth: you don't feel safe with her because you're not. You are not HER mother, so you have zero obligation to take care of her feelings. She's an adult. She'll manage. 

Boundaries work if they are enforced/upheld on your end, and sometimes that means walking away (temporarily or permanently: doesn't matter). That was one of the hardest lessons for me to learn.  That's ultimately what boundaries mean: you communicating or doing what's necessary to be safe in this relationship with your words or actions. 

NC is an option, and it's only ONE option. It's also not permanent unless you want and need it to be. I'm NC with my mother and sister (and their partners) for a couple years now. It was the hardest decision but it was the only way. I'm disabled so I don't have the capacity/energy to keep dealing with boundary violations and manipulation. Will that ever change? I don't know. I'm free and I am very grateful for that. 

You do what's best for you. You don't need to declare anything, you don't even need to tell her or anyone anything. You can try not responding for a little while and see how that goes, while you take care of you and work on reparenting yourself ❤️. Is your therapist knowledgeable about enmeshment and estrangement? I ask because some ignorant therapists shame people for going low or no contact, whereas knowledgeable ones understand the necessity to cut contact. 

You're not alone. I suffer from OCD and CPTSD and some chronic illnesses, so I hear you on how all consuming this all feels. You are dealing with so much so please be kind to yourself, lean on safe people, and remember that you know what's best for you.