Be critical enough to learn, while keeping enough optimism to keep going.
Doubting yourself can be soul-crushing. A little self-criticism can help you see what needs to improve, but too much of it starts to drain the energy you need to do anything about it.
It is important to stay critical and honest with yourself. It is just as important to keep your spirits up and remain positive.
The point of reflection is to help you move forward, not to convince you that you cannot.