I've played for half an hour and left. The next time I opened the save was nowhere to be seen. I left by pressing V and going back to the menu.
Anything I did wrong? How can I prevent that from happening again?
Saves are broken and jank of extreme proportions. For a sliver of a chance, try saving your current game, closing it and loading it, that
might cryptically bring your old save, instead of the one you made now. Not certain, but possible.
As a whole, the game is shit in both mechanics and systems as a whole, so trying to tough it out to the end is best done as fast as possible and in a single sitting, so the game has the least amount of chances to either jank you out or shit itself (or bore you, as the game consists solely of spam and long, unskippable animation timeouts).