I certainly seem to remember this woman I was with.. with whom I'd shared 1500 texts in a month.

And if you think that I was responding to her every message as soon as I got it, you're wrong.
The trick to carry on conversations is to know when to keep them going and when to let them just be.
As a simple rule, use the full stop(.) as the termination point of the conversation. Yes, nor "..." neither "??", if you want to drop the conversation off at a point,end it with a statement followed by a full stop.
The keywords being "statement", not any open ended line, not any teasing comment, not some question. A statement is the flattest form of communication.
e.g.
Her : So, it was crazy, right??!!
You : Yes, it was. Now, I'm off to studies.
There. You said it. You let her know that after this point she shouldn't bother you or even if she does text you, it's not going to be replied back to. End the conversation with a valid reason for non-communication past that point and you won't face her "shutting off" like you put it.
Also, fix a schedule. If you're too much in love, call each other once a day, everyday (given you cannot see each other). If you're just getting to know each and enjoying it a call every other day for an hour, works. If it's just another woman you've met, or you're not that invested, call her once or twice/week. This was an example. You can of course, fit your own variation with whatever works for you.
Good luck.