Those decorations look a little sharp. The betta might be catching its fins and tearing them. Run a piece of nylon over the decorations, if it snags on anything then remove the decoration.
Clean water will help with healing up so up the water change frequency. You could consider adding some aquarium salt to help prevent any infection setting in. 1 rounded tablespoon per 5 gallons, if you see no improvement after a week, double the dosage. Remember to add in more salt in proportion to any water changes.