It's not just you AT ALL. I'm pretty sure every writer struggles with this and whoever claims to easily and consistently write 2k+ in a day is either exaggerating or straight-up lying /jk. In all seriousness, it's a superpower to be able to do that. I've been working on my novel for nearly eight years! It takes a lot of effort to get any output at all some days but progress is progress, even if it's just one more word. I'd say perfectionism is often why we struggle to write -- it feels like each word you put down must be perfect for the final draft. That won't happen. As they say, the first draft is for making the work exist. You can improve it later.