How do I deal with being good at a job I find pointless?

You are good at something you cannot bring yourself to care about, and that split is wearing you down. The skill is real. The emptiness is also real. Doing excellent work on something that means nothing to you is its own kind of tiring, and you are right to notice it. The competence is not the enemy. It is proof that you can master hard things when you choose them. The problem is only that you have not yet aimed that ability at something that matters to you. That is a direction problem, not a you problem. Start by naming what you would care about, even in a small way. You do not have to quit tomorrow. Spend one hour this week on work, paid or not, that feels like it matters. Let yourself feel the difference. That feeling is the compass you have been missing.