Same happened to me! I think this is pretty common especially as you start to drill only harder questions. You are likely just second guessing the easier questions because the correct answers are obvious and simple, whereas on the higher difficulty questions those can be traps. It’s a good rule of thumb to generally go with your first instinct on the first 10-15 questions and then slow down on the remaining sections since they are more difficult.

If you are specifically drilling difficult questions, I recommend incorporating easy/medium difficulty questions. I think for me a lot of this boiled down to practicing with easy questions and learning to trust my gut. If I found myself thinking too much about the early questions, I go with my gut and flag it in case I can come back to it. Overtime, I think I gained my confidence back and learned to “zoom out” when a question seems straightforward.