Depends on how debilitating it is, if its bad enough, therapy might be a useful option.

Body dysmorphia about weight might need a bit more help than you can give as an individual.

It might not meet the criteria for that, but worth consideration.

Edit: to clarify, dysmorphia like this is where the brain refuses to acknowledge the relatively objective reality of a “normal” weight.

It’s often one of the underlying causes of bulimia/anorexia and the converse.