Everyday I try to tremolo it almost feels that the reason I am unable to sort of control it better, getting in and out of it is because I didn’t learn it when I was younger. I can haphazardly get it going eventually, sometimes I can do it super impressive one day, and the next it takes me some adjustment to do it. The earlier in the day I can do it easier, the later in the day is when it gets a bit tricky. All of this I am speaking of is the pure wrist motion, using no elbow, completely relaxed loosey-goosey tickly blood flow, floppy muscle feeling. The turning gypsy wrist angled wrist motion, not the back and forth one. I have to really move the wrist to get a big stubby 3.0 through the string though, and it feels like it has to be the tip and at a certain angle. I try not to look sometimes cause I feel like I will lose it if I try to think about it. 
And its not like I can go all day long or forever, once I get it going its good for about 15-20 second. But if I try to go again, no way, it just isn’t going to be near as articulate, and it might not even happen. I have been working on this well over a year now, about a year and a half. So it takes time to develop this, or it can so don’t give up. This is all one speed, I dont dare try to control how fast or slow. It is as fast as my body can gyrate my wrist, no more no less. If I had to adjust speed or possibly even dynamics, cause it can be quite tricky to dynamically adjust volume for this one sometimes i can but it is not easy, I would utilize another motion to tremolo.
I think I would consider this like one of those just go for it motions. Don’t psych yourself out, the motion you are doing is the correct one. It just takes time for the body to figure out how to align things right to get it to sound. Like learning how to play hacky sack might be a way the body is trying to figure it out. You just fail over and over, but dont think I am not doing it right. If you can take one of those really thin dunlop picks the paper thin ones, angle the wrist and do the really fast chord thing that most beginners can do, this is the motion, but you gotta just keep trying. Don’t over analyze to much, just try to do it everyday. And then forget it and move on to learning something else. Trying to do it on one string, is why it is so tricky.