Three things this can turn out to be

Depression. Low mood for most of the day, most days, for two weeks or more, with the things you used to enjoy no longer landing, sleep and appetite changed, and a heaviness that does not lift when something good happens. A comparison loop does not do that. Something else is doing that.

Disordered eating. Restricting, purging, exercise that has stopped being optional, weighing yourself repeatedly, or eating being organised around how you feel about your body. That needs assessment and it needs it early, because these get harder to treat the longer they run.

Body dysmorphic disorder. Hours going on mirrors or photographs, either checking them or avoiding them, a part of your appearance you cannot stop thinking about, and life getting smaller around it. That is a specific and treatable condition and it is not vanity. It also does not respond to reassurance, which is why the people around you telling you that you look fine has never once helped.