This happens to me all the time. It's weird how we can tell what words going to be a problem before we have even tried to say them. I guess it's like what others say that we have remembered in the past that we've had problems saying something so then winter to pay having the same problems again. I changed my words sometimes do whatever i can get out. Obviously this is not the best tactic as not all words you can substitute plus it just reinforces the behaviour. I am trying to avoid substituting no just people I've told me you'll never get better otherwise.