Lower the pressure to about 12-15 psi and thin the paint so that it will flow properly at that pressure. Get some old plastic milk jugs or soda bottles for practice. Start at about 6" from the surface and learn to control the trigger so that you get good coverage. Several light coats work much better than one heavy coat.
The key is practice. That is absolutely, unquestionably, unarguably the only way to learn to use an airbrush properly. You have to develop techniques that work for you. If 20 psi at 12" works for you then go with it. I don't spray that way but I've learned what works for me. I like low pressure (never more than 15 psi usually more like 10 to 12) and in close (never farther than 6" usually more like 3").
The trigger on a double-action airbrush is one of the most valuable tools you have. It allows you to constantly vary the paint flow as necessary. Learn to use that freedom. Watch how the paint hits the surface and you'll start to see problems before they happen. But the only way to do it is practice.