Sadly you must lose all the items on your character... but you can keep playing on the same save by accepting this loss and doing this:
When you are looking at the save from the main menu, there is a "reset slot" button, which when clicked, allows you to choose to reset the events and objects, vehicle, dirt, etc., You must use the reset option to reset the player stats. If you lose any unique items from your inventory being deleted, you can go get more in a different save, and put them in the mail box, which shares an inventory across all saves.