Mouth sores are hard to heal because the mucus membranes keep everything so moist and wet that it's hard for the blood to fully clot. My son has a bleeding disorder called hemophilia, and when he gets mouth bleeds or hematomas (an area where blood has collected under the skin or tongue tissue like a bruise--it sounds like the sore your daughter has), then we give him a clotting medication for a few days. The clotting medication is specifically for the mucus membranes and is called Amicar. You don't have to have a bleeding disorder to use it, but you do need a prescription for it. I would also recommend letter your daughter suck on ice or popsicles frequently until it's healed. The cold helps the blood to clot. Most importantlly, I would recommend having your daughter tested for von Willebrand disease (vWD), a bleeding disorder that is actually quite common. They're now finding that 1 in 50 people have it, but it has only been getting public recognition for the past few years. Hemophilia on the other hand is rare and almost always it's boys that get it, so your daughter wouldn't be a candidate.) If your regular pediatrician is hesitant to prescribe the medication for you or have your daughter tested for vWD, then I would contact your local Hemophilia Treatment Center (they treat all bleeding disorders there and test for them). The website for the Denver, Colorado one is http://www.uchsc.edu/htc/programs.php
Also the website for the Hemophilia Society of Colorado is http://www.cohemo.org/
They should also be able to provide you with information and resources about bleeding disorders. Getting your daughter tested for a bleeding disorder early will save you and her a lot of potential grief--especially when she gets older and starts her period, etc. There are medications to treat the specific blood clotting problems of these disorders. If your daughter has had a mouth bleed for that long, I would definitely check into it. Good luck!