is your system's natural defense mechanism kicking in. Anything new or unknown introduces stress -which is what you are feeling. You have played your whole life thinking about and directing your swing. The one swing thought that you play your best with..."don't break down"...it's a comfort, it's familiar. You play great golf, why change?
So when you try something different,anything different than the norm, whether it is clearkey or whatever, your system does a big "WTF is going on here?" and your defense system kicks in to protect you from the threat. This is perfectly natural and is the way we are wired.
I think all players who try clearkey struggle with the process when first introduced to it, some more, some less. You know- fear of the new and unknown. I know I did. That is why in my book, I have the student from the very first exercise do the drills using their clearkey....in this way they begin to become acclimated to playing without thought - the same way they are gradually being acclimated to the correct moves and positions physically through the exercises. A person must learn to swing correctly, but I believe it is just as important that they learn how to play without thinking about it. My book does this incrementally right along with the exercises and drills so that by the time the student has built the swing, they will be very comfortable with the clearkey process - it wont feel "awkward" to them.
As far as the one thought being a help to a fine swing..."pretty is as pretty does"...most people would tell you why change? However, it is known that consciously( by words, commands,suggestions etc)interfering with an activity that is able to be performed unconsciously or automatically will result in less than optimal performance. And that is the reason a clearkey must be thoughts that are totally unrelated to the swing, it's execution, the outcome, anything even remotely connected to the swing.