Other people have already given a lot of great advice, so I'll just add this bit: Working on someone else's codebase—especially if it's an open source project

Other people have already given a lot of great advice, so I'll just add this bit: Working on someone else's codebase—especially if it's an open source project with many different contributors over a long period of time—is its own distinct thing that has to be learned. Everyone has to learn how to do it, regardless of where their abilities as a programmer are. If you are adept (as it sounds like you are) at building your own software, then it can feel like the rug has been pulled out from under you ("Am I actually any good at this?") when you try to contribute and simply can't grok what's going on, but if you persist, you will pick it up. Don't take your struggles now as a reflection of your abilities.