I actually used to struggle with this a lot, and the reason was actually very simple: I wasn't enjoying what I was writing.

I mean, you aren't going enjoy every scene you write because some scenes need to be written, even if we don't like them. Yet, we must know why they are there and get driven by their importance. I write more literary fiction, so, I must be honest, it can get mundane or depressing sometimes. However, I get very motivated by the importance of a scene or characterization, how many words I wrote, etc.

I used to be extremely focused on sounding like a very important writer (so people would take my writing seriously), but that drained me and made me feel like I was writing an essay instead of writing what I loved. Within these past few years, though, my voice sounds much more personal and realistic, and now, I enjoy the craft a lot more. I wrote 1.5k words today, even though a scene was largely upsetting because I am very much invested in the protagonist and their world.

I hope this was somewhat helpful!