Consciously make an effort to fill your mind with positive thoughts. Practice focusing on the hundreds of positive aspects of your life. Be aware of your ability to see, talk, walk, hear, watch tv, what humans have accomplished, etc.
Thoughts always keep racing through your mind, so gently keep your focus on all the positive details of your life. Realize that you are the one who chooses what thoughts to dwell on. Choose those thoughts which enhance your life.