I did have a problem with disappointed teachers too.
My piano teacher would have been livid if I got merit for grade 5 but when I came out of grade 7 I thought I did so badly that she was prepared for the worst... so she was actually pleasantly surprised when I got merit- so was I. It was my first merit- always distinction before- but it didn't sting as much as I thought it might because I was already so sure I wouldn't even make a merit that I was just overjoyed.
If your teacher's

that you got merit in grade 6, I find that rather strange. Even having reached the "post grade-5s" is an achievement!
Don't worry- I'm sure you'll do great if you just stop worrying about what your teacher thinks.