How do I cope with becoming a manager and losing the work I was good at?

You were excellent at engineering, and you gave it up for a job you were never trained to do, and now you feel mediocre at both, because you no longer write code and you are still learning to lead people. On top of that you are grieving the craft you loved. All of that at once is a lot, and it makes complete sense that you feel lost. Feeling like a beginner again after being an expert is disorienting, but it is not proof you made a mistake. Every good manager was once a bad one. The skills are real and learnable, they are just different from the ones that made you a great engineer. You are not mediocre. You are early. Give yourself the same patience you would give a junior engineer on their first project. Grieve the code honestly, because it was a real love and it deserves to be mourned. But also notice the new craft taking shape: growing people, removing obstacles, building something bigger than you could build alone. That is worth learning to love too. Pick one part of managing to get genuinely good at this quarter. You are not less than you were. You are becoming something new.