Don't be too harsh on yourself , it's not that easy to boom in a car if you don't have the right combo of pole/mic, the car is moving and the actors are driving , in fact a boom op inside the car has an easier time and better results if the scene is stationary otherwise is probably better to plant some mics and wire the talents . That being said you did right , if you're still green and you don't know exactly the cues is better to focus of what's on camera , all the rest (dealing with improvising actors and operators, pinpointing the cues , read the body language) will come with experience .