Recurrent mouth ulcers can be caused by various things-
1. Trauma- an ill fitting denture or recurrent cheek bite.
2. Aphthous ulcers- recurrent
3. Recurrent herpes
5. Auto immune diseases- Bechets, SLE
6. Vitamin/mineral deficiencies
If rest of you health is good, it should not be a big problem like HIV.
Remove any traumatic cause if present. Maintain oral hygiene.
Change you tooth brush to new and softer one.
For pain you can use anesthetic-antibiotic gels.