But we have to assume we'll overcome obstacles and not be crushed by them.
There's a big difference between not letting knowledge of obstacles deter us and assuming we'll overcome them. The former is very important, for just the reasons you state. The latter effectively ensures that we will not overcome said obstacles, because we won't have effective plans to do so.
I think this whole thing is purely an exercise in identifying obstacles. Identifying obstacles is a necessary first step in the process of determining whether or not you can overcome them.
The second step would be analysis. It's pointless to identify obstacles just so you can assume that you'll overcome them all. Not all obstacles are created equal. You need to take the time to look at the more major ones in detail, so you can create a strategy, and then assess that strategy. The end result is more accurate assumptions.
Of course your best-laid plans may go to waste, and your best-made predictions may prove inaccurate. But to quote Dwight Eisenhower: "Plans are worthless. Planning is essential."
There's a big difference between not letting knowledge of obstacles deter us and assuming we'll overcome them. The former is very important, for just the reasons you state. The latter effectively ensures that we will not overcome said obstacles, because we won't have effective plans to do so.