I always think of challenges in therapy as being opportunities for me to grow and change by learning or by trying to do something new or different, combined with my T's belief that I can do it even though I feel very anxious, apprehensive, and uncertain of my ability at the time. But my T's belief and confidence in me propel me forward to a place where I am willing to try because he knows me well. I trust him and I know that he would not push me to do something unless he knew I would have an excellent chance to succeed, boosting my own self-confidence and helping me learn to challenge myself. My T has challenged me to do things that I would never have tried on my own because I would never have felt capable of doing them if he hadn't challenged me. It's been my experience that the times when I thought he was absolutely nuts and had expectations that were far too high were actually the times when I gained the most by trusting his judgment.
I think if you have a T who knows you well enough to have a good sense of your abilities, then being challenged is a wonderful, albeit frightening at times, opportunity for growth. It has been enormously helpful in building my self-confidence and has brought me closer to my T as we share in my successes and learn from my failures.