Low memory appears to be "shrinking." Can't free up memory no matter what apps and files I delete.

I got the message about a month ago and deleted an app or two and thousands of photos. Then I got the message again a week later without downloading anything or taking more than a couple hundred photos. So I delete a thousand more photos and more apps. The alert comes back a day later, and is now there constantly no matter what I get rid of. I can't even share photos or open my gallery without being prompted to delete an indeterminate many more. What do?