A few things that I am thinking about is how she wrote this. This is when you put a puppy down. I haven't received one of these in years. And I make sure that I am not treated like this. I can see her patting you on the head and calling you a good boy.
Don't reply for now. You need to reflect how you got here. Why is she sending you this? Why is she talking to you like a harmless child. Like her little puppy. At what point did she stop respecting you? Were you needy? (I know you. I know you usually can't wait to send shit to girls. You can be desperate and needy. And from this text I feel like you might've been). So first thing you want to do is step away from the phone and really go through the entire relationship. Identify where you went wrong and what you could do differently next time. You do your own reflecting. Do not feel the need to message her. You don't owe her that.

After you've calmed yourself and done your own reflecting. You've pin pointed particular situations where you went wrong. And why you are being treated like this.
You can send her a message. I'd honestly wait a week. This isn't exactly a saveable text.
I'd want to know exactly where you went wrong. What she is feeling and thinking. And not this woman talk that she just gave you. Not this "it's not you, it's me" nonsense.
Find a way to open her up. Accept her decision (she isn't expecting this btw).
"Haha no worries! You too!" -- send that. Nothing more.
With a bit of time, a couple weeks (there's a good chance it annoys the fuck out of her that you took it so well. I bet she becomes suspicious that you had another woman) and ask her honestly, tell her it's for honest feedback - it won't hurt you, where exactly you went wrong. You've done your reflecting so you have some ideas what happened. You can straight up ask her. Ask her for honest feedback. Tell her you want to improve with women and relationships for the future. Don't hold back. You can even threw her some easy ones. Tell her you know that he was too sweet or nice. Too needy. She'll probably, even lightly, agree with this. This will open her up a bit.
Getting this feedback and simply moving on will help you particularly.
Now what this could do is that she sees that you are not actually needy. You can walk away. You may even have another woman already. Women HATE when men are indifferent to them leaving. It bothers them. They need you hurt. Angry. Emotional.
When women threaten to leave or get another man, I don't play along. I don't engage. I don't get emotional. If anything I'll encourage it.
"well that's unfortunate! But you have to do what you need!" and move on. They NEVER EVER move on.