I've felt like this at every company I worked for, and not just at startups. At the start, I'm learning tons of new stuff, solving new problems, growing a lot of lots of different ways, and that gives me energy to put in way too much work. Eventually that slows down; I've seen those problems before, it's just another feature on some thing I've already worked on, and it starts to feel repetitive.

I just need something new every once in a while. I thrive on change, apparently. (My brother doesn't; he's been working at the same company forever.)

The problem is: I never get around to my own projects; when I started at a new job, all my energy goes into that. When it turns into an old job, that extra energy is gone. I never have the surplus energy for my own stuff.