How to get the months that have not yet been paid

Currently, the yyyy-mm that were paid are stored in a lookup field named Month Paid For.
For instance, we only paid the vendor until 2022-11 even though we were supposed to pay them as of 2023-2. How do I obtain the months that have not yet been paid? ( 2022-12 , 2023-1 , 2023-1). Thanks !