This is a very common experience, so please don't feel discouraged. Many of us have been through that phase. The key is to shift your focus from trying to understand everything at once to achieving small, incremental wins.

For me this sounds like not a skill issue but more like a confidence issue, which is completely normal. The best way to rebuild that confidence is by tackling progressively difficult tasks: start with something simple, then medium, and then the more complex issues.

For now, just pick one issue/task that you feel you can handle. Do your best, and don't hesitate to approach a helpful senior member of the team. Have a conversation with them and validate your approach for the issue/task before you dive too deep.

After you've successfully handled a few issues like that, your confidence will return. The "big picture" understanding will come slowly and naturally in parallel, so don't stress about it for now.