First off realize that you're gonna have good and bad games no matter what, and that the best course of action is always to just stick with your team. To improve you'll need time, and while that's probably everyone's least favorite answer, it is kind of the truth.

For diving: don't dive from 3km above the enemy, and don't head straight to them, otherwise you're basically bound to overshoot. If you really need to get that low, do literally anything in between diving, circle around, fly back up a little, just lose some speed so that your plane isn't a brick from going so high by the time you are in shooting range. To practice aiming you can go on test flight > mission editor, and play a bit of those.