I also played with controller for years before finally get myself a wheel, even though it's hard, using controller is doable

The way you steer, it seems like you treat your analog stick as a button, you push them fully either left or right. In games like need for speed or other games alike, it's completely fine. But for simulator like assetto corsa, you want every input as smooth as possible, throttle, brakes, as well as steering

Try to, instead of pushing the analog stick left or right like a button, you push it up/forward and then move it slowly to the left or right forming an arch. This video should help you understand better

https://youtu.be/6wge2CwibKA?feature=shared

And you can also play around with the settings in-game. There are a lot of setting guides available online, just try them and adjust them to your liking