Are you focused too much on work? When is the last time you took a week or two off just to mentally reset? How is your social life? I was interested in a specific field a few years back, but I had zero friends or connections in said field. I started a meetup group around the topic, grew it to 1500 members in just over a year, learned a TON about the field in the process, and made invaluable and exclusive connections that would have been otherwise very difficult. It was a beautiful blend of social and professional advancement and I highly recommend something similar.
Last point: if you're considering grad school, be aware that this is much more accessible and palatable early in your career vs. late. If you have a shot at getting into an MIT or Stanford, why no give it a whirl? It isn't necessarily the degree that is of value, but the high-end network you'll obtain in the process.
Keep your head-up -- motivation will ebb and flow throughout your life. This is normal and a sign that change is in order.