Hi.. Thanks for the query.. I have gone through your query and can understand your concern.. As per your complain Canker sores in mouth can occur due to causes like Vitamin B 12 deficiency, acid reflux, eating plenty of sour candies or other foods, oral infections, digestive problems etc. On the other hand yellow coating on Tongue commonly occurs due to poor maintenance of oral hygiene leading to accumulation of debris and dead cells over the tongue papillae along with bacterial or fungal overgrowth. I would suggest you to start cleaning your tongue using a tongue scraper.. You can Gargle with 1:1 solution of hydrogen peroxide and water and also dip a toothbrush in this solution and gently clean the tongue.. Chew hard foods like carrots and apples as they have a cleaning action in mouth.. For Canker sores you can take Vitamin B complex supplements, gargle with 1:1 solution of milk of magnesia and Benadryl.. Apply Lidocaine gel over the sores to relieve pain.. Hope this information helps.. Regards..