Yea solving hard problems can be quite enjoyable, but can easily become a burden on your mind/body over time. When you're clearly making progress, then everything feels wonderful even if it's challenging. But when you're stuck, when your code/model isn't working as designed, if there is a clear gap between your current state of understanding and the amount required to solve the problem effectively, then you can fall quickly into despair. It's made worse when you see others doing similar work to what you do, but being far more successful at it. And made even worse, when you tie parts of your identity to the problem you're trying to solve, and the code you write. But that's the nature of what you're trying to do, so it isn't all bad.