Setbacks happen. One moment you are pressing forward, making great strides towards your goal. The next moment you find yourself slamming into a brick wall that appeared out of nowhere, at high speed. The impact throws you off your feet and onto your back. You suffer injuries and lose your momentum. Instead of moving forward, you now have to cope with this turn of events.
At best, it is a minor irritant. At worst, it can be a devastating blow, which you may not recover from quickly enough. But what matters is not what happens; it is dealing with it that counts.