Update checked listbox items

I have a checked list box on a windows form that I need to update the items that are checked.   The checkedlistbox is already populated with predefined items.    I need check the values in the list box that have been previously selected by the user.  The previously selected values have been stored in a comma seperated string.  As an example, the CheckListBox has the following items

Value1
Value2
Value3
Value4
Value5

When the user last updated the record, he selected Value1 & Value4.   Those values were stored in a database field as follows: "Value1, Value4".    I now need to first check to make sure Value1 and Value4 still exist in the checkbox list.  If they do exist I need to show them as being checked.  If they don't exist I need to add them to the list and then show them as being checked.

I am hoping that is possible but cant figure out how to do it.