Why can't I delete my old text messages?

I'm trying to get rid from stuff from my iPhone, but most of my memory is the documents and data.

I thought I could get rid of it by deleting some of my old text messages, but it won't let me delete any ot my old texts.

Every time I try deleting a message from like a month ago my phone freezes and then whe I try to exit my messages

and reopen it, it deletes all of my messages! PLEASE HELP!