How do I get over being scared driving a big rig?

You can drive, but a big rig in tight spots and bad conditions is a different animal, and being scared of it is not weakness. That fear is your mind taking the weight and size of what you are moving seriously. It is worth listening to, not ashamed of. Fear shrinks when skill grows, and skill grows only through repetition in the hard spots. Practice the tight maneuvers where it is safe, slowly, again and again, until your hands know them without panic. Break the scary thing into small parts and master them one at a time. Confidence is built, not summoned. Go slow and use every tool you have: get out and look, take the extra minute, ask the veterans how they read a tight dock or a bad grade. No good driver was fearless at the start. They just kept doing the hard thing carefully until it stopped being frightening. Yours will too. Give it time and honest practice.