I had that happen a few years ago, and lost a TXT that was really precious to me, from Al.

I need to find some way of backing up TXTs. I have some from my Brother, from before he died, that I really don't want to lose.

Your phone has Bluetooth and you have phone software for it. You can connect your phone to your computer and then using the software that came with the phone you can hook into the phones TXT database and back it up.
I do it all the time with my Mobile phone. I have a Nokia 6233 and I have done the same with my Sony Erisons.
All new phones Allow this. You can either use a Cable or Bluetooth if youur computer has it.
You can buy cheap bluetooth modules for your computer.

I checked back through my phone to see if I had any saved messages that I had sent you, but all I seemed to be able to find was SMSes saying "cock".