Have you ever had a bad dream? When you dream, the mind perceives it as real. If you wake up in the middle of a scary or bad dream, you find that you are perspiring, your heart beat is faster and you are scared. Even though the dream is illusory the body and mind reaction is real.
You sit up and may be have a sip of water. In a short while your body and mind gets back to a neutral state. You know that the dream is not real and you are able to get over the discomfort caused by it. There is no real damage done.
Let us say that you have an important meeting tomorrow and you are hoping that it goes well. You get anxious about the outcome of the meeting. Rather, you are anxious about the probability of an unfavorable outcome. When your mind pursues this line of thought there is obviously some body-mind reaction happening. You feel uncomfortable and there is some negativity. When this happens your mind takes some sub-conscious decisions.
The perceived negative outcome is like a bad dream. It is not real, but your body-mind has reacted in a real way. When you get into the meeting with this state of mind you are contributing to the outcome in a negative way.
When you wake up in the middle of a dream, you immediately realize that it is not real. How do you wake up from a perception?