This, more than anything. It's easy to get bogged down in a large code base and lose sight of what you're trying to accomplish. Cue the anxiety at that point, and watch your tasks become even more difficult. Your note that "it's all just code" is absolutely perfect: you just have to trust in your experience and knowledge. Figuring it all out from there is just a matter of tracing things out a point at a time.
The really cool thing about that is that it's really no different than how we handle other new experiences in our lives. Nobody walks into something new and just knows what to do. Without even consciously realizing it, we make observations and draw inferences as we relate those observations to previous experience that allow us to make better-informed judgments. And thousands of years of evolution have made us rather good at this.
You already have the skills to jump into the deep end without being immediately overwhelmed, it's just a matter of applying them successfully.
The really cool thing about that is that it's really no different than how we handle other new experiences in our lives. Nobody walks into something new and just knows what to do. Without even consciously realizing it, we make observations and draw inferences as we relate those observations to previous experience that allow us to make better-informed judgments. And thousands of years of evolution have made us rather good at this.
You already have the skills to jump into the deep end without being immediately overwhelmed, it's just a matter of applying them successfully.