What to do when you lose your appetite from nervous overeating of junk?

Don't eat junk. There's nothing to do if you already ate junk. Obviously you should avoid it in the future. If it's just a bad habit, stay conscious of what you eat, don't have junk food at hand (try healthy snacks instead), and have family/friends support you. If nervousness is the underlying problem, check the many healthtap answers about how to handle nervousness or anxiety.
Find out the ''why'' Either ''nervous overeating'' , a need for junk food --a way to not feel something you wish to avoid, and particularly a loss of appetite; each is sufficient reason for an expert psychiatric evaluation. The underlying reason(s) can run the gamut from strictly physical, e.g., diabetes, adrenal or thyroid diff. To purely psychiatric to a comb. Eliminate any bodily contribution;then ref. 2 a psych.