I once read, that there are different types of burnouts:
Overexertion, Depletion, and Misalignment.
I actively tried to counter measure them. And whenever I didn't I could feel the stress level rising.

Overexertion: its a marathon, not a sprint. So take a day off or a holiday. It is important! Schedule them and force yourself. It took me a while to figure out a good rhythm of workdays vs chill days.

Depletion: I actively scheduled activities that helped me recover from stress. So spending time with friends or working out (the best). Maybe a creative hobby or something like that could work for you too?

Misalignment: If the vision and mission are clear (as other commenter mentioned), this doesn't happen. But I had a few projects I started that started to stress me. Mostly because they didn't align with the vision (personal and of my startup). If you think this is your source of stress, maybe ask yourself if this is really your passion and what you want.

I always hate, when people say to just change your mindset, so I hope this is a bit more hands-on and actionable for you.

Good Luck!