I have a sore the outside corner of my mouth where lips meet. It's been there for about a month. It reopens everyone I open my mouth, help?

Mouth Sores. The most common cause is "angular chelosis" and can be a fungal or bactierial infecction easily treated with an anti-fungal ointment. Itcan also be related to vitamin deficencies or othe more serious disorders. It also can be relted to an allergic reaction to lip gloss.
Perleche. Saliva, yeast, and sometimes bacteria, can all contribute to this problem. I usually have a person apply an anti-yeast cream, like clotrimazole, miconazole, terbinafine, etc., to the area after meals and at bedtime. Use sparingly and rub in gently. Especially at bedtime, it's covered with plain old greasy vaseline -- to protect the area from saliva overnight. Usually this works within 2 weeks.