I felt it in the past. My experience was related to the environment of the startup I was at. I started it with 2 others but we were all remote and our ideas didn't bounce off well with a sense of energy and electricity. But the people I am working with now are quite different, they love what they do and we get energy and clarity from our interactions. Grinding without seeing results is difficult, so try to get back on the "effective" grind. If the product is built and you are only maintaining code you won't grow as much as you desire. Start a hackathon project! Get back the excitement by testing/working on the mission with a new angle (perhaps incorporating AI? It's pretty interesting to learn-- not sure if it fits nicely into your startup)
Bottomline without too much advice. We feel that we are growing when we are learning and acting deliberately. When we are challenged and stretched to do new things. An idea is to find a feature/idea that needs to be validated to ship an early version of it in front of your customers to test. It could start a new adventure.