"Don't sacrifice what you want most for what you want now. Write down what you want most and see it often"

What i think peter is trying to say is that don't do things that you want at that moment because they will mess up what you really want. Don't do things that will mess up what you really want for yourself in the future. Peter set a goal for himself and didn't let anything stop him from achieving that goal. If you want something in life you should st your mind to it and don't let anyone take that from you.