Great picture!
I am sorry to hear that and I know exactly how you feel.
I ended up in saving png backups time by time.
Sometimes each layer (with alpha STRG+S). Takes ages, but at least once before closing a project.
It's teadious and no fun, but beside working on smaller canvas with less layers, it's the only advice I have.
As verve is not warning you, I also tried to test my system before doing serious projects.
Increasing canvas, adding layers, painting a bit on it and save project, close and reopen.
Crazy and stupid, but once you found your systems limit, you are a bit more safe.
While I was using verve often, I made this tests. Didn't find my thread about it.
It's all about your graphic card.
Good luck!