There is no word "bursted". The past tense and past participle of the irregular verb "to burst" are "burst": "the filling burst out" "the filling has burst out"

I have some cultural questions for you. In the United States, "bread" would normally mean bread only, with no "filling." Bread filled with slices of meat, lettuce, cheese, etc would be a "sandwich." But when squeezed, the filling of a sandwich would not "burst" out. It might "fall out" if handled by a baby. If you tell me that the filling "burst out," it suggests a sweet pastry with a jelly-like filling. "When the baby grabbed my jelly doughnut, the filling burst out."

Rather say: 'grabbed my sandwich, the filling oozed out.'