Dealing with problems that you don't know the answers to right away is part of the job, and it's one that stresses you out. By doing extensive prep, you're avoiding some number of stressful incidents, but you're also making sure that the incidents you do have are higher stakes.
You might try doing a less detailed read of documentation before you start, then if you get stuck, you can expect to look at the documentation to get you unstuck. Maybe it will, maybe it won't, but you'll practice getting unstuck and it will hopefully be less stressful.
Some of us just despair on the inside.
If you’re still junior in your career, just pay attention. If you’re getting senior level but you’re not getting enough work, start doing more personal projects that require you to “own” the whole thing from end to end.
Have you read the seminal literature (the thing they didn’t teach you in ugrad)? Gang of Four OOP design patterns, Patterns of enterprise architecture?
You might ask your employer for a subscription to O’reilly books online.