How do I stay proud of behind-the-scenes work that goes unseen?

You already stand on solid ground: you know your work is the foundation, and you hold its worth without needing it seen. That is a quiet kind of strength, and it is rarer than you think. Most people cannot feel proud of anything no one claps for. The risk is not that you will stop believing it. The risk is slow erosion, the day after day of being overlooked wearing down what you know. Guard against that. Being seen by yourself is a real thing, and it needs tending like anything else. So tend it. When you close out good work, mark it in your own mind as good. You are not asking permission to matter. You already decided. Keep deciding it, and the lack of applause will stay what it is: other people's blind spot, not your truth.