Your best shot is all you can give. If you get through the selection process and on a course you still only have a roughly 50% chance of gaining a rating. That's the long term stats, plus or minus, but if you don't have a crack you'll never know.
Many days are fairly mundane with everything on the rails but occasionally you get a day with, say, extensive bad weather around (like this afternoon) and you well and truly more than earn your pay. All you can do is keep plugging away, keeping them apart and in some semblance of order. It might not be pretty but everyone gets where they're going in the end. It's also very satisfying because you've worked bloody hard, done the best job you could and hopefully made it a bit easier for the next controller down the line.