Positive thinking has a billion clichés about glasses being half full, rainbows and all sorts of cute things. While wonderfully helpful as a child, when we become adults we must convince the rational mind to adopt this philosophy in a more direct and logical way. Take the Law of Attraction out of this argument as this spiritual aspect does not speak directly to the mind and is more of a symptom of a healthy mind than the cause of it.
If we only take control of what we are in control of we would eliminate pretty much everything but thoughts. As this is where your true power and control of your physical life is formed. When the mind is under stressed thoughts it goes into fight or flight mode, releases high doses of the cortisol (a fat storing and physiologically damaging chemical released by the body) and will cause physical and mental havoc on your mind and body. This is why stress seems to age a person very rapidly and cause a very pessimistic and jaded attitude towards life.
The good news is; this is a choice and you can choose to start looking at the glass half full and start seeing the rainbow in the sky if you choose to see it. It may take some time as you probably have lived your entire life not keeping track of better feeling thoughts but when you start to do so, it will get easier as time goes on until it eventually becomes second nature. Then your mind and body will become much healthier. And if you use the Law of Attraction to your benefit, this is when your manifestations will come into your physical existence.