Freud taught us about the pleasure principle and the reality principle. The pleasure principle controls us when we choose the easy solution first and then pay for it with feelings of discomfort, e. g. if we buy a bar of chocolate we feel well for a while, but later pay for it with an increase in weight and perhaps regret.
Compass for a better life
Sartre said that our lives are formed by our choices. It seems that you have chosen the pleasure principle a little too often. Try to be aware of those moments when you choose, and regularly take the more difficult short-term way out. Exercise instead of looking at TV, buy an apple instead of a bar of chocolate, etc.
Don't expose yourself to unnecessary temptation. Make use of your natural tendency to take the path of least resistance by having wholesome food easily available and be sure to make it difficult to get junk food.
Even after a week you will have more self-confidence when you see that you can really improve your lifestyle.