She may have received the first text, but may have been working on opening the large file, so she didn't respond right away, and then time passed and she got sidetracked about it. Large files/apps can be slow to respond, particularly if for some reason you are in a bit of a poor reception zone. I don't always sit around and wait for them if I'm in the middle of other things.

It sounds like you have an attentive therapist who slipped up once. Try not to be completely literal in that every single text will be replied to. I know that's what she said and how you took it, but the reality is, very few of us respond to every single text we receive. And, if we receive many texts, it's easy to lose track of what we've responded to and what we haven't. A little wiggle room for your therapist is probably in order.

I would certainly bring it up with your therapist if it continues to bother you.