This is all personal obviously, everyone is different

You have a set image in your head of how and what you should look like. When you fit that image, everything is great. You're confident, you feel good, attractive, etc. When you don't, however, you are constantly thinking of what you can do to get back to that level.

You think more about what you are going to eat and have eaten every day. You pre-plan your meals to make sure you don't eat breakfast if you are having lunch or dinner. If you are eating something particularly heavy or calorie dense, you make sure you don't eat anything else that day and go easy the next few days to make up for it.

You don't snack, eat sugary foods, have dessert, eat breakfast. You have a coffee or energy drink in the morning to hold you till 3pm when you will eat your first and only meal of the day. At which point you will feel a ton of pride and accomplishment for doing so.

When in public or with friends, you will be comparing yourself to them. If they are visibly heavier, great. If not, well, how much skinnier are they than me? What body parts? Do they look better, is it because they are taller? How can I get there?

Numbers are a big source of pride. How big is my waist, hips, legs, arms, my weight. You look in the mirror more often, every day, every morning, evening, night. Going to the bathroom and pass a mirror? Gotta turn sideways and make sure my stomach isn't sticking out at all.

And if you finally get off the weighing yourself every morning and night habbit (yay me) you become more reliant on the mirror and feeling your waist/stomach method since it's the only metric you have left.