There are a few things that you should pause and self-evaluate to determine if it's actually burnout.
Do you feel different ABOUT writing while you're doing it? If so, then it could be burnout. If you don't, then it's unlikely.
Are you working through continuity issues? A problem that is easily confused with burnout is complexity. When you start writing a story, anything can happen. As you get further and further in, there are more and more internal continuities you have to consider. This slows down writing a lot, because you have to remember what someone did dozens of chapters in the past. These stack up more and more over the course of a story, which slows pace down a lot.
I think a lot of authors think they're experiencing burnout, when in actuality, they're just struggling with maintaining the continuity within their own envisioned universe.
As mentioned, writing outlines, or taking a break can help, and actually helps with both of these. Even when you take a break from writing, you're likely not taking a break from thinking about your story, and you'll hammer out a bunch of those internal continuity issues by thinking about them over time.
The prescribed cure for both tends to be the same thing, though I'd argue you can just power through the continuity issue, but powering through burnout is a no-go. Recognizing which one is the problem can help you develop as a writer too.
A lot of people say, "I've started a lot of stories, but I burn out after a few dozen chapters, and then move on to the next project." I think (and this is just conjecture) that many of those authors are running into the point where they have to consider continuity within their own story, don't want to, and so they drop it. That's completely fine, but understanding that it isn't the dark specter of burnout, but is actually just something every story that isn't episodic experiences can help you make the educated decision on what to do.
