Girl flakes last min, blames my texting?

Two things:

1. idk if you was texting badly, would need more context of what happened before but
2. you shouldn't have sent that text so promptly. that kind of signalized you reacted emotionally to her flakes, what is precisely the wrong move when girls flake/shit test. I'm not saying what you said is wrong, it is actually right, but i think on that stage we must be able to be a little more unnafected by the girls actions. You could have answered her with something like "am I?" and let her explain herself more, what would put her in a position of vulnerability, since she would have to be the one giving explanations.

Now, with regards to what to do now, I'm not advanced, so my hint should not be taken as golden rule. But I think the best frame would NOT be saying things like "don't worry". Since she's already the one sorrying about, maybe using the frame to troll her might be a good option. What I mean would be to respond something like: "Yea, you messed, let's see what you can do now to fix it". But as I said, I'm no advanced, so it'd be better to take more opinions.

go with "its okay. Hypothetically speaking if we were to make plans again, what is the likelihood of you showing up"