First off, you've done a wonderful job on a relaxing little game. I see it's something you come back to slowly over the years and I love that. I hope to see more updates from you!

Now about that healing. I tried the Sauge berries since the description mentions animals eating them when sick, nothing. I tried sleeping multiple times on a full stomach in my cozy fire-lit cottage, still nothing. What am I missing?

Thank you for playing, I'm eager to go back to developing it :)

In order to heal, you must rest, like, really good rest. Well fed, comfortable and warm. It may take some time so it's best to try not to get unwell on the first place :)

Are your in winter? using a bedroll or a proper bed is the best way to rest your tired back.

This is just speculation, but it feels like being sick drains your health gradually, while eating Sauge and the rest of the foods that make you feel stronger combats that by regenerating your health gradually, which may or may not outpace the drain. However, if you can't sleep comfortably, the maximum amount you can heal up to takes a hit and you see your health bar stopping recovery further and further from your actual max health. To fix that you'd want to sleep on a bed or a bedroll while the temperature is at 15 or higher, then eat a Sauge when you wake up.