Hi @Melissa717 I hope you are well!
It would be unfair for you to cancel bookings you have already accepted in favour of a longer-term booking.
If you cancel them then you'll have the dates blocked in your calendar so your longer term potential guest wouldn't be able to book them anyway- airbnb put this system in place to prevent this very issue. (as well as losing your Superhost status, and facing financial penalty)
Could your long term guest not book elsewhere whilst you honour your current reservations and then make his booking to start at the next available point in your calendar?
Best wishes
Paul.