I have a scar under my chin. It hasn't healed in 9 months. What should I do?

See your doctor. Non-healing of wounds is a problem we can usually take care of. Of greater concern would be any possible associated health problems of which you may or may not be aware.
Vitamin E oil. On your scar daily, also make sure you're eating well and consider taking antioxidants to help the healing process. Skin requires vitamin c and zinc, good fish oils can also help.