@Mrs-G0
You are responsible for the prices and discounts you set in the listing. Smart Pricing can not "screw" you, as it is you who is setting the minimum / maximum price.
As a new host better not start with "long term stays" (so set maximum stay to 27 nights).
Did you ask Airbnb to cancel the reservation penalty free ? ("I am a new host and made a mistake" sometimes will convince them to do so)
Did you ask the guest to cancel the reservation within 48 hours after booking ? (then guest would receive full refund)
Only Airbnb can make the dates available, maybe contact them again, as 3 months blocked on your calender is a severe penalty for a new host !