typically when I get past the second or third line of composing a message, it crashes and I lose the entire message that I had composed, and it has started doing this while after my first line.
Welcome to Android Central! Try going to Settings>Apps, select Facebook Messenger, then Storage, then Clear Cache/Clear Data, then Force Stop. (Be aware that this might erase any messages you might have saved -- if you don't want to lose them, then only do the Clear Cache.)