Thinking positive is a decision you make to concentrate on the nice things in life and not be dominated by the destructive things.
You must remember that in everything there are positive and negative impacts. Some thoughts can make you feel good and others can spoil your day.
If you consider the possibilities ahead of you, the nice persons in your life, some of the nice things you have heard people say to or about you, it will lift your mood.
This does not mean that you will not experience the down side. But you must remember that the thoughts that you form are yours, and subject to your control.
If you want to exercise more control of your life, you must not dwell on those experiences that will bring you down. Make up your mind what you want to do about those negative realities.
You should not ignore them, but rather give yourself time to consider what about them has such a sway on you. Then think of what you are able to do to overcome them, replacing them with positivity.