The ego's only perspective is based on survival. Survival is the exclusive focus of the ego, therefore thriving is not our natural default and our ego will interject and sabotage our efforts toward change. Yes, this includes health related changes that might actually cause you to survive for longer. Your ego can only make decisions based on the past.
From a biological perspective, we have evolved to conserve energy so that it is there for us to use when we need it. Our wiring essentially is pulling us in the opposite direction than where we need or want to go. Our society has created foods that are tasty and have done a great job marketing them. We collectively recognize these brands and consume them regularly.
Most prepackaged snack foods are convenient, cheap, and widely available. Until recently, we didn't have much information about the health effects of such foods. With more information we are realizing how some of our food and health choices have determined where we are today. Knowing the negative health consequences usually doesn't stop us from consuming these products and doesn't incentivize us to exercise more.
This explains why most diet/exercise routines fail. WE ARE ATTEMPTING TO FIGHT A WAR AGAINST OUR OWN CONDITIONING. Willpower is like the big bad wolf; it works for just long enough to convince you that it works but when the ego has had enough, it steps in a sabotages your efforts.
If you see diet/exercise as a source of pain or discomfort then you will move away from staying with it EVERY SINGLE TIME. This will play out over and over again without you even realizing it unless and until you work with your unconscious mind.
You looking good and feeling good is not on your ego's agenda and never will be. Working with what comes up, the resistance you face, is the way THROUGH resistance. You can't casually walk around the obstacle that is stopping you or it will get you again and again. You must move through it, wrestle with it, understand why it is there, and there you will emerge with a different perspective that will never stop you again.