The right kind of optimism.

When you start out you are naïvely optimistic.
You're fantasizing about how each project you start is going to be huge.

But then, one after the other, your projects fail. You realize that you kind of suck. You become a pessimist, maybe even a cynic. Seeing a project fail hurts less when you were predicting its failure all along. This isn't a great stage, but it's closer to the truth. You are wiser now. But you are also more miserable.

The third and most mature stage is when you realize that this is how the world works. Only one of maybe 100 things you try is going to work. And yeah, you do kind of suck, but not more than any other person. It's ok. Everybody struggles. The whole point is to just keep going and suck a little less with each iteration.

When you accept this fully, you can be an optimist again. A mature one. You don't believe that each project you start is going to succeed, but you absolutely believe that one in the next 100 will. If the project you're currently working on doesn't succeed - that's alright! You just came one step closer to the one project that will work. It might be the next one.