What these phrases do is put up mental barriers. Be careful when you say this to yourself and other people. It's amazing what this negativity can do the creative process. If anything you should try and do things that are hard and you should try to be in over your head. When you put yourself in a situation where you know very little, this is the situation where you learn the most.
When things are too easy you are not learning anything. Also when you put up these barriers you forget to experiment and try new things. When you mix different disciplines together magical things can happen. Don't setup unnecessary mental barriers.