I was brought in as CTO to fix some stuff. I fixed that stuff and then realised that how I wanted the business to operate and how the CEO wanted to operate were very far apart.
It all came to a head when somebody got fired and I disagreed with both the decision and how it was carried out. This was just one item amongst a bunch of other things.
This drained me so hard emotionally. I was CTO at another startup for much longer and never got close to this level of burnout. I've talked to someone else about this (talking is helpful) and they had a similar experience. It's not always about the hours or the tenure. 60 hours/week in the right environment is very different to 40 hours/week in the wrong environment.
It took a few years for me to get over this. I took some less complicated jobs with less responsibilities and less personal investment before chucking myself back into the fire. It's going well so far and I'm definitely more resilient as I've been through some pretty dire work shit since then.
Sorry to hear you're having a tough time of it. Getting less personally invested for a while really helped me. I also took on some hobbies that I had total control over and that helped too.
Good luck :)