Hello @Agnes54,
Unfortunately, you are stuck. Those dates will remain blocked. Airbnb has a pretty firm policy about cancellations by hosts. Hosts are penalized for cancelling, and one of the penalties is blocking the dates for future bookings. The main goal is to prevent discrimination, but also to promote guest confidence that a booking is really a booking. The other penalties are (sometimes) a fine, and (always) becoming ineligible for superhost for a year.
With extenuating circumstances (floods, fires, major illnesn), these penalties will be forgiven, but you will be stuck. Just preferring to rent to someone for a longer time will not work.