It can be hard to accept that there will be people who are naturally very skinny, but your focus should be your own health. Not just skinniness, because skinny doesnt mean healthy and neither does underweight. Your focus should be healthy for you and your body. Stop comparing yourself to others, wondering why others can still be very skinny... Everyone is different and hating on others is not a good idea. Everyone looks different and have different body types if your body type is not naturally very lean (which only x% of the world naturally have) then dont try to force yourself to maintain that body shape. It is also important to remember that after a restrictive eating disorder you often need to reach a BMI of 20-23 before the body starts to function properly. Though some maintain and are healthy and the body works fine at a BMI of 19 or 20, those are still healthy weights but generally, the body has been through so much and been very stressed for a long time so it needs to weigh more to feel safer after the eating disorder. Though that is also a generalisation, everyone is different. But also areminder, dont focus so much on actual numbers such as weight or BMI, those things aren't so relevant.... it really doesnt matter if you are a BMI 18 or 22, the important thing is that your body is healthy and so is your Mind! Because it is your thoughts and behaviour you need to change as well.... you need to overcome your fears, overcome the things holding you back and controlling you. Fight your triggers and the ED thoughts.
Focus on what is best and healthiest for you. Stop comparing yourself to others and hating other people because they look a certain way. Instead, learn to love yourself and not feel the need to change yourself. Not feel the need to be the skinniest or be the "best" or most lean. Just focus on being a healthy and happy version of you which you can live with.