Hi. Thanks for the query.. Yes, your symptoms are pointing towards Angular Chelitis that can appear as rash or cracks at the corners of mouth.. It can occur due to a number of common causes like Iron deficiency or Anemia, fungal infection,ill fitting dentures, loss of teeth leads to folds of skin at the mouth corners,systemic diseases like Coeliac disease etc.. In your case, as you wear night guard at night and that too is too old seems to be a probable cause for the Angular Chelitis and you should consult your dentist and get evaluated and he can assess the mouthguard and also the cracks over the mouth corners to look for the exact cause.. Treating the underlying problem that is causing the symptoms will help in its resolution.. You can also apply a thick coat of Petroleum jelly over the corners to relieve the dryness and smoothen the area.. Do cool compresses.. A low potency steroid ointment along with antibiotic and antifungal combination ointment can also be advised.. Take multivitamin tablet for 15 days.. Drink plenty of water.. Hope this helps.. Regards.