Switch teams, ideally to something entirely different than what you’re doing now. Google makes it easy and painless (in most cases) for a reason. I stayed at my first team at Google for years longer than most people do, and though I started strong, by the end I found I was feeling similarly to what you describe. I took that as an impetuous to switch teams and moved to doing something very different, and now a year later I’ve got my fire back and I’m learning tons every day. I’m sure at some point I’ll get comfortable and complacent here again, but now I know to keep an eye out for it and take that as a signal that it’s time to move forward again.