Whenever we have a thought- any thought!- it fires off neurons. Fire off the same thought repeatedly and pretty soon our very efficient brains start creating links and pathways. Our brains figure out what we find important enough to repeat and make it easier and easier for us to access these thoughts. These become known as habits or reflexes.
It is actually a physical changing of our nervous system to accommodate what we access the most. Our priorities, self talk, and beliefs actually get hard wired into our bodies over time. The more we practice something the more nerves get dedicated to serving that purpose.
I used to say "I'm old" in a joking kind of way, not knowing how destructive this was. I don't want to feel that way. What a terrible way to reprogram myself!
So now I wake up everyday and say "I am better and better every day in every way" because that's the reality I want my body to support. Since I have started taking control of the way I think my world has changed.