You wouldn't think that one skinny string would mess with your mind so much, would you?
I agree that you should spend time with the 6 only for a while until you get comfortable on it. Once you get a decent comfort level you'll be able to bounce back and forth between 4's, 5's and 6's pretty easy.
Most players that are dedicated 6+ string cats approach playing ERB's with a different mindset. If you stick with it and it does become comfortable for you, you'll find that your whole approach to playing will change .... hand positions, voicing, patterns, etc will be quite a change from your lesser string basses.
Starting out, try playing everything at the 3rd and 5th fret positions without using open strings. This will help you get adjusted to playing patterns and positions across the neck rather than up and down. Not suggesting that this is the case for you, but if you play 6+ string basses thinking like a 4 stringer with 2 extra strings you won't be getting the full benefit of the additional range.
Like I said above, it's really a change in mindset ....
