The hardest part was actually post-launch. During development, the game received almost unanimous praise and at least a couple dozen awards. After release though, while it did receive a lot of 9s and even some 10s, the critics that didn't like it hit me hard. While in hindsight it's obvious--no game is for everyone--the unanimous praise prior to release set false expectations for me. Now, the game's been a huge success and sold a lot of copies, so whenever someone doesn't like it, I can have the relaxed "Well, it's not for everyone" response, as opposed to taking it personally.
Launch was incredibly stressful. The finishing touches, final bug fixes, five platforms (8 separate SKUs which needed to go through certification) as well as marketing, PR, the trailer, and so on left me ill and on little sleep. There wasn't enough time to do everything and as I juggled so much, the game had to came out of the oven before it was finished cooking. While I was aware of some rough edges, I hoped that everyone would look at the overall package, but instead those were cherry picked and held up to the light.
It was a big learning experience. Your game's only as strong as its weakest level. I dwelled on the lower review scores and spent another month polishing the game to address the gripes they had. Now, I'm incredibly happy with the game and it's what I intended it to be in the first place. However, that didn't change those existing reviews which with the current state of the game should be higher than they are.
They represent an old version of the game but are still up on the wall. They definitely damaged things creating a "mixed reviews" stigma, but perhaps through word of mouth, sales picked up a couple months after launch and the game became a great success, generating even more revenue this year than last. With so many balls up in the air, it's impossible to know the exact reason for this, but the oddest thing is that it launched in October, and the lowest revenue was in November. It's be higher ever since.