Actually not word substitution, I never substitute words unless it's by accident.  Sorry that was a misunderstanding, I say it feeling so different that it "feels like" word substitution after I back out of it and try again but it's always the same exact word.  Because the word just came out like any other, on autopilot, as if it was substituted from the word I thought I couldn't say yet it was that same word.  I just didn't think twice about it so I didn't think of it as an impossible wall but rather something getting in my way so I just had to sidestep it so to speak.  Just pulled back and went again and it came out like any other word.  No substitution neccessary.