How do I deal with resenting that my mastery is invisible to everyone?

You can design a die for anything that exists, a mastery earned over years, and almost no one who benefits from it will ever know the skill it took. The parts just appear, and the genius behind the tool stays invisible. That resentment is understandable. It is hard to give your life to something excellent that the world never sees. But consider where the worth of your mastery actually lives. If it depends on others recognizing it, it will always feel thin, because most true craft is invisible to those who use its results. The alternative is to root your pride in the thing itself, in knowing that you can do what few on earth can do. That knowledge does not dim when no one claps. It is yours whether they see it or not. Still, invisible does not have to mean unspoken. Teach a younger worker what you know; let one person see the depth of it. Take on the problems only you can solve, so the mastery gets used at its full height. The recognition may never come from the crowd. Let it come from the work, from the few who understand, and from yourself, who knows exactly what you are.