Overthinking isn't simply thinking too much but thinking about things in a disproportionate manner. You spend longer than you should on things that shouldn't require as much mental energy. Let's say you want to teach yourself New Testament Greek and you're faced with lots of choices about the textbook you want to use. The fact is, there are a lot of beginning grammars that will get you to where you want to go, provided you apply yourself. These textbooks represent the plethora of right choices, some better than others perhaps, but you're still going to get to your goal regardless of the grammar you use.
Just make an informed decision and then go for it!

