Change is scary for most of us and going from a size 18 to a 6 is a big change. We don't have our fat to hide under anymore. I am not scared of getting thin ,but I can sympathize with you because I hate change. I know change is needed and is good and healthy but I hate it.
Anyway just take it slow and steady and don't think about it a whole lot and just live one day at a time. You will do fine when you get to a size 6 all your worries will be gone and you will just be this new thin and healthy person that you were ment to be.