How do I teach one struggling student who isn't getting it?

You know your subject cold. What eludes you is bending it to fit one specific student who is not getting it. That is a different skill entirely, and knowing the material can even get in the way, because what is obvious to you is a wall to them. The gap between knowing and teaching is real, and closing it is its own craft. The key is to stop explaining and start listening. When a student is stuck, the answer is not a better explanation from you. It is finding out where exactly their understanding breaks. Once you see the precise spot they lost the thread, the fix is usually small and clear. Next time a student is stuck, ask them to walk you through their thinking out loud, step by step, until they hit the point where it goes wrong. Do not correct, just find the break. Then teach only from there. You will spend less breath and reach them faster, because you will be fixing the actual problem, not the one you assumed.