A sushi chef would be a poor choice to teach you about cooking fish!

But I understand. It really just takes practice.

Try a handful of different recipes with the same fish and you will start to get a feel for it. Then try a handful of recipes with a different fish. Etc. After getting the hang of a few of them, you'll be more comfortable with judging how and how long to cook the fish based on the filet or steak that you're working with--how thick or delicate the meat is.

For me, I was wary about chicken for the longest time when I first started cooking, afraid that I would undercook it. Same thing. It all just takes experience.