Praise be to Allaah.
You have to try hard to fast the number of days which you think you most likely missed, and ask Allaah to give you help and strength. Allaah says:
“No person shall have a burden laid on him greater than he can bear.” [al-Baqarah 2:233]
Try your best to be on the safe side and fast the number of days you think is most likely that you missed. You also have to repent to Allaah, and Allaah is the Source of strength.