How do I deal with being indispensable but invisible at work?

You built the whole thing, and somehow that made you both essential and unseen. Everyone depends on you and almost no one notices. That quiet resentment is understandable. It is hard to be the reason things work and never be the story. But consider what invisibility really means here: it means your work is so solid that people forget it takes anyone to do it. That is a strange kind of praise. Still, you are allowed to want to be seen, and pretending otherwise only feeds the resentment. Rather than wait to be noticed, name your contribution plainly to one person who should understand it. Not as a complaint, just as fact. You are not chasing applause. You are refusing to disappear, which is a fair thing to want.