You just have to keep doing it and eventually you get better at it. You'll NEVER be as good as keyboard/mouse but you'll eventually get good enough to play with everyone else doing the same thing.

Turn on aim-assist in the options and don't feel bad about it. The open secret is that every console-based FPS does some degree of aim-assist. They have to as the input just isn't precise enough. Turning on aim assist actually just adds a little more. It's never actually off.