Actually quick horses seem hard but they actually better than those who need lots of legs! Two advantages mainly:
- they don't refuse. It's better a horse pull you to the fence than you push him to it
- they auto calculate their strides ! So they only relies on your steering mainly and few strides before the fence they adjust and lock to it.
- u need to be faster in your jumping positions
- if it's go wrong in the steering it goes wrong in the strides for them
So just trust him, and also he needs to trust you!
I have gone through the same exact thing many times before! I started by talking to my trainer about everything short-term we need to work on before the upcoming shows. Then I rode as much as possible and only focused on what I am showing on and what my trainer told me. After that we quickly got better in time for our show. I suggest you do the same thing and dont be afraid to ask questions on things you feel like you are struggling on! Stay confident and hopeful and remember everything you learned - because it will end up helping you the day of the show! Good Luck (and get some sleep)!!