What a great thread; I love all your suggestions and if you follow some of them as a combo Baxter, you'll be serene, even if you are awake. For me, warm baths are almost magical, and the music, at a very low volume is great too.
I have a version of the serenity prayer which is, 'I can't control other people, I can only control my reaction'. It's so evidently true that I it calms me very quickly.
You know, writing a letter can be a great outlet, but I suggest you don't send it because you need to let go of the feeling you can control your daughter if you could only find the right words. Let go of trying to fix her; it's very liberating.
Instead, think about little things you can do for your grandchildren, without galloping in to rescue them. Just be a spot of calm for them to talk to, and only offer advice if they ask.