Recently I was talking to another founder about why starting a company is so hard. She said, "You are just doing everything for the first time. Even once you figure something out, you iterate, so everything is new."
It reminded me of being a new mom. Everything is new. Once you figure something out, your baby (and situation) changes.
Just recognizing why it was so hard made me feel better and show myself some self-compassion. Hopefully it helps you a bit, too.