I made a huge mistake

Hi,

I work for a stylist (unpaid internship) and today spent the entire day running her errands. One of my jobs was to drop off a suitcase at one of her friends (let myself in to her house) I let myself in and dropped off the three suitcases I had been carrying around the city. As I left I was sure I closed the door behind me however I received a message from my boss saying ‘you left her door open’. I’m hugely embarrassed and this is shamefully not the first mistake I have made during this internship. Does anyone have any advice? I’m humiliated.