You are what you think and so shall you be.
Come up with 5 positive thoughts for every negative and know that what you focus on will increase.

You can easily re-phrase a thought. Here is an example:
Instead of thinking "If I do not exercise I will get fat". Think, "I exercise to be healthy" Why? Because the first thought has a focus on not exercising and getting fat.

It's that simple, even if you initially do not believe what you are thinking, anything you repeat becomes a habit you act on.