If this is only happening in the messaging area of your phone, and clearing the cache did not help, your next step will be to delete all of your text messages. If you have several hundred, can can delete thread by thread, starting with the smaller ones first. If you try to delete them all at once, the phone , may become unresponsive.
I have the same problem, texts that were loading fine this morning, are suddenly not loading. After re-booting, I was able to get small threads to load (as big as ~250 texts). before I saw the reply from tmo_alicia_b above, I decided I'd try to delete threads, and that I'd start at the biggest (~7000 texts). Now it's just sitting, with the spinning wait icon reading "Deleting" instead of "Loading"
So now I'm stuck.