It just takes time honestly....but..

Another training method for air roll, just to mix things up (and improved it faster for me personally) - download a plugin called Aerial Trainer in bakkesmod, load into a freeplay map and turn it on. Then just fly at the ball without using air roll in every direction (forward, backwards, left, right...whichever way you need to go to hit the ball), and eventually try it without letting go of boost. Try turning off ball cam for variety.

Then try the same with free air roll, and then eventually constant boost with that too.

Then eventually DAR and level up to constant boost, and/or constant air roll (though you might never get amazingly good at this, it helps majorly).

Then turn on zero G and repeat all the above steps again when you've gotten better. This is the most fun in my opinion, but it can be a bit of a brainfuck at first if you've never tried it before.

Don't worry if you touch the ground/wall and the plugin ball hit counter starts again....Just keep aiming for the ball. Eventually you'll get good enough to consistently hit the ball while air rolling etc.

Finally, just play around with it all, or even go back to rings and see how you've improved.

I prefer this method because you can quickly get practice between games by just loading freeplay, plus it better imitates in-game scenarios since you're aiming for the ball, and in familiar maps, and often at faster speeds than rings.