You're doing great! Many of the commenters here are saying this is normal and you shouldn't be worried. They're right. Relax. You're asking questions and learning... don't worry about not learning fast enough. No matter how quickly you learn, it will never be "fast enough"!

Here's some advice which may help. Focus on smaller problems. If you need to solve a problem with an internal tool with poor docs, tell yourself you're not going to solve the problem right now. Right now, you're just going to improve the docs. So dive into the tool and focus on one method at a time. Read the source, the tests (if they exist), and update the docs with the knowledge you gain. Stay focused on each individual aspect of the tool until you're an expert on it before moving on.

This way you'll be in a position of ever-expanding authority, you'll inevitably arrive in a position where you can solve the original problem easily using the internal tool, and you'll have created some docs which will be useful to other people on the team!