The trouble with saying you don't want to text about work or blaming your mental health, is that it opens the door for her to continue messaging you, but about personal stuff. It also blurs the boundaries even more, so instead of creating distance, she could see you as becoming more intimate.
Similarly, if you say you don't want to text in the evenings, she might think it's fine on the weekends.
Texting this could help:
Taking a break from technology for a while. Catch up tomorrow!
If she sends you loads of messages, you could mention it, eg:
Wow! That's a lot of messages! Let's catch up tomorrow.
After that, I would just not reply and if she mentions it just say something like, you're not a big talker/texter.
One of my loved ones likes to talk (too much sometimes). I've been known to say things like:
"It's nice to be quiet sometimes, isn't it?"
"Look over there!"
"Oo! Hang on a sec!" (and then silence).
You need to have a certain relationship/vibe for this sort of humour though.
Another idea is to start wearing headphones/ear protectors when you're working.