How do I feel challenged at work again?

At the start, everything was new. Every day taught you something, and that constant learning had its own electricity. Now you are good at what you do, and very little is genuinely new anymore. The competence is real, but the buzz that came from always growing has faded, and nothing has moved in to replace it. What you are missing is not the difficulty of being a beginner, it is the growth, and those are different things. You do not actually want to be lost and clumsy again. You want to feel yourself expanding, and somewhere along the way mastery quietly closed that door because you stopped seeking the edge of what you cannot yet do. The stimulation did not leave. You stopped going toward it. So go and find a new edge on purpose. Learn a skill adjacent to your work, take on a problem you do not yet know how to solve, teach someone else and discover what you only half understood. Growth is available at every stage, but after the early years it stops arriving on its own and has to be chosen. Pick one thing to be a beginner at again. That old electricity comes back the moment you stand somewhere you cannot yet do the job.