Here's a tip, and once you understand what I'm talking about it's going to go a very long way.
Look at how much my head moves in relation to your video. Our cameras are mounted at nearly the same spot.
The reason why your inputs are "jerky," and this includes throttle and brake applications, is because right now you're reacting as a driver. You get up to your braking zone, and you react to it. You jump on the brakes. You turn-in. You jump back on your throttle as you get to each of your "assignments."
In order to go fast, you need to be looking far enough ahead where you're no longer reacting to what's happening in front of you. You're looking at places that you will be in 5-10 seconds, not where you will be the next second. As you get up to your braking zone, even before you start braking you should already be looking at your exit to the turn. That way, it'll give you the opportunity to figure out where and how much to apply the brakes, where and when to turn-in, and where and when to get on the throttle even before you enter the turn.
Once you're looking at the right spots, and the sooner you're looking at those right spots, and the further ahead you're looking, at those same right spots, the faster you will go. Don't worry about braking, how smooth you need to be, and all that. It will come naturally as soon as you start looking further ahead.