How do I stop feeling overwhelmed by flight navigation and systems?

When the navigation and the systems all seem to demand attention at once, the overwhelm is real, and it is not a sign you do not belong up there. It is a sign you are still holding every piece separately in your head, which takes enormous effort. That load eases as the pieces start to connect, and they will. The reason it feels like too much is that right now nothing is automatic yet. Every setting, every readout, every step is a fresh conscious act. Experienced pilots are not managing more than you, they are managing less consciously, because the routine parts have become second nature and freed up their attention for what actually matters. You are not slower. You are just still doing by hand what will soon run on its own. So stop trying to master all of it at once and drill one system at a time until it is boring. Pick the one that rattles you most, learn its normal flow cold, then move to the next. As each becomes automatic, the fog lifts a little more. Overwhelm is not a verdict on you. It is just the stage before the pieces click together, and the way out is one system made routine at a time.