This is a interesting topic, as my mind can trick me into focusing on what I don't want. Today, I realized that I fell into the same trap yet again. I had to laugh out loud when I saw it.
We want to focus on what we want. What do we want? To play faster, louder, more fluidly, more confidently, with more joy. What do we focus on? - not making mistakes, not getting tense, not cracking notes by playing too loudly. You cannot get what you want by focusing on what you don't want. It may seem to be a subtle difference in language, but in terms of results, the difference is tremendous.
Children don't seem to have these issues as much as adults. Many people think children pick up music quickly because of certain physical attributes, but I believe it's because most children have less critical minds. They make mistakes and it doesn't distract them from feeling good when they play. They don't take the mistakes personally, so they are not fearful of making mistakes while they go for what they want. This is incredibly important to understand.
We adults tend to think that we can have joy in playing only when we are playing well. This is the critical mind at work. The paradox is this - to play well we must feel well inside. If you want to play open and freely, you must feel that way inside. As my teacher has told me many times - go after what you want and feel well no matter what befalls you. How simple, and yet how easy it is to forget this.
The critical mind is a clever salesman, but for today, I will not buy.