Dont respond emotionally. Remember you are the strong one here, she is the one in turmoil. I dont know that you need to respond right away at all. If you feel like you do, maybe just acknowledge that you received the message. Her decision is not about you, nor any reflection on your worth - she is doing this for her and her own reasons.
Maybe go out and go for a bike ride, or do some yoga, or get some exercise. Do whatever it takes to clear your mind or at least settle it. Breathe.
W 34 Me 42 Married 7 years together 8 0 kids 1 beloved dog BD 4/6/2018 I moved out 4/7/2018 I moved back in alone 8/05/2018 I file 3/06/2019 D official 5/7/2019